BREEAM V7 SURFACE WATER MANAGEMENT CREDITS | 50% RUN-OFF IMPROVEMENT

BREEAM Version 7 introduces the most demanding surface water management requirements in the scheme's history, fundamentally reshaping how developments must handle rainfall and run-off. The headline change—increasing brownfield run-off improvement from 30% to 50%—represents a 67% increase in performance expectations that will challenge conventional drainage design approaches and require substantially larger SuDS infrastructure on constrained urban sites.

Beyond the percentage increase, Version 7 restructures surface water credits from two to three separate assessments: run-off rate control, volume reduction (entirely new), and water quality treatment with enhanced monitoring. This shift reflects updated understanding of surface water flooding mechanisms, recognition that rate control alone doesn't address annual water balance issues, and growing evidence that SuDS performance often deteriorates without proper long-term management.

For development teams, these changes mean surface water management can no longer be solved through standard underground attenuation tanks meeting statutory minimum discharge rates. Version 7 strongly favours nature-based SuDS providing visible water quality treatment, volume reduction through infiltration or reuse, and designed maintenance regimes verified through monitoring. Projects relying on buried tanks with minimal treatment will struggle to achieve credits, regardless of compliance with building regulations or planning conditions.

Overview of BREEAM V7 surface water Changes

BREEAM V7 surface water requirements build on the SuDS hierarchy and climate resilience principles from Version 6, but with substantially enhanced expectations across all aspects of drainage design and long-term performance.

Key Changes Summary:

50% Brownfield Improvement: The threshold increases from 30% to 50% improvement in peak run-off rate compared to pre-development conditions. This applies at both 1-year and 100-year return periods with climate change allowances.

New Volume Reduction Credit: Third credit introduced requiring demonstration of 30% reduction in annual run-off volume, or maintenance of pre-development infiltration rates, or rainwater harvesting capturing minimum 50% of roof run-off.

Enhanced Water Quality Requirements: Treatment effectiveness must be modelled and demonstrated, not just appropriate treatment features specified. Post-construction verification and operational monitoring become mandatory.

Updated Climate Change Factors: Post-development scenarios must apply +40% peak rainfall intensity (increased from +35% in some Version 6 applications), with +45% in high-risk catchments.

Maintenance Verification: Maintenance agreements alone insufficient—must demonstrate maintenance is actually occurring through monitoring data and inspection records submitted at post-construction assessment.

SuDS Hierarchy Strengthening: Greater emphasis on infiltration and surface features, with more rigorous justification required for underground attenuation or piped discharge.

Whole-Life Performance: Assessment considers 60-year operational lifetime, requiring demonstration that SuDS will continue functioning effectively through multiple climate change scenarios and maintenance cycles.

Mandatory Minimum Standards: Excellent rating requires minimum 2 surface water credits, Outstanding requires all 3 credits. This makes surface water management non-negotiable for higher ratings—cannot trade credits elsewhere to compensate.

These changes collectively add 20-40% to typical surface water management costs and require 30-50% more site area dedicated to SuDS features compared to Version 6 compliant schemes.

Credit Structure Version 7: Three Separate Credits

Version 7 splits surface water management into three distinct credits, each with specific requirements and assessment criteria.

Credit 1: Run-off Rate Control (Peak Flow Management)

Objective: Reduce peak discharge rates during storm events to prevent downstream flooding and reduce pressure on drainage infrastructure.

Requirements for Credit 1:

For Brownfield Sites:

Demonstrate 50% improvement in peak run-off rate compared to pre-development conditions. This must be achieved at both return periods:

• 1-year event (100% annual probability) - frequent storms

• 100-year event (1% annual probability) - design standard

Pre-development baseline calculation:

• Assess actual historic drainage infrastructure and discharge arrangements

• Use appropriate run-off coefficient reflecting surface materials and drainage

• Cannot assume 100% run-off unless site was genuinely fully impermeable

• For derelict sites vacant >5 years, professional assessment of historic drainage required

Post-development calculation:

• Include all proposed impermeable surfaces (buildings, roads, parking, paving)

• Apply +40% climate change factor to rainfall intensity (+45% in high-risk catchments)

• Model attenuation features reducing peak discharge

• Demonstrate discharge rates 50% lower than pre-development at both return periods

Version 7 Enhancement - What Changed:

Version 6 required 30% improvement. Increasing to 50% means:

• Attenuation storage typically increases 50-80% in volume

• More site area required for surface SuDS features

• Underground tanks must be substantially larger

• Discharge control mechanisms (hydrobrakes, orifices) must restrict flows more severely

Example Calculation:

Pre-development (brownfield car park):

• 1-year event: 8 l/s discharge

• 100-year event: 35 l/s discharge

Version 6 target (30% improvement):

• 1-year: 5.6 l/s maximum (30% reduction)

• 100-year: 24.5 l/s maximum

Version 7 target (50% improvement):

• 1-year: 4.0 l/s maximum (50% reduction)

• 100-year: 17.5 l/s maximum

This typically requires doubling attenuation volume compared to 30% improvement—substantial infrastructure increase.

For Greenfield Sites:

Peak run-off rate must be no greater than pre-development greenfield run-off rates. Assessment methodology updated:

Pre-development greenfield rates:

• Calculate using IH124 methodology (Institute of Hydrology Report 124)

• Updated 2024 version includes climate change in baseline (new requirement)

• Soil type, vegetation cover, and topography inform calculations

• Typical greenfield rates: 1-5 l/s/hectare depending on soil permeability

Post-development rates:

• Must not exceed greenfield rates at 1-year and 100-year events

• Climate change (+40% rainfall intensity) applied to post-development only

• Requires substantial attenuation on most greenfield sites

• Infiltration-based SuDS strongly favoured to achieve rates

Version 7 Enhancement - Updated Methodology:

The updated IH124 methodology (2024) produces slightly higher greenfield rates than previous methods, providing marginal relief. However, the requirement to apply climate change to post-development (but not pre-development baseline) typically means 40-60% more attenuation storage needed compared to Version 6 approaches.

Assessment Boundaries:

Run-off rate control assessed at each discharge point from the site:

• If single outfall: assess at that point

• If multiple outfalls: each must meet requirements independently

• Cannot compensate high discharge at one point with low discharge at another

• Internal site drainage routing must be clearly shown

Evidence Requirements Credit 1:

Hydraulic calculations:

• Pre-development run-off rates (methodology stated, assumptions justified)

• Post-development run-off rates with climate change applied

• Attenuation storage volumes and locations

• Discharge control specifications (orifices, vortex valves, hydrobrakes)

• Routing calculations through SuDS features

Drainage drawings:

• Site layout showing catchments and discharge points

• Attenuation feature locations with dimensions

• Pipe network with sizes, gradients, inverts

• Control structures and overflow arrangements

Software outputs:

• MicroDrainage, InfoWorks ICM, or equivalent professional software required

• Flow-time diagrams showing attenuation functioning

• Sensitivity testing demonstrating performance under different scenarios

Justification for discharge rates:

• If discharging to sewer: sewerage undertaker agreement for proposed rates

• If discharging to watercourse: EA or IDB consent for discharge

• If rates exceed recommended thresholds: robust justification why this is necessary

Climate change documentation:

• Confirmation of +40% (or +45%) rainfall intensity applied

• Reference to EA guidance for catchment-specific allowances

• Justification if lower allowances used

Credit 2: Volume Reduction (Annual Water Balance)

Objective: Reduce total volume of water leaving the site annually, not just peak rates. Addresses groundwater recharge, prevents cumulative downstream impacts, and promotes water resource management.

This is a NEW credit in Version 7 - did not exist in Version 6. Recognises that controlling peak rates doesn't address annual run-off volumes, which affect:

• Groundwater recharge rates (important for aquifer replenishment)

• Baseflow in rivers and streams (ecological impacts)

• Combined sewer overflow operation (CSOs discharge when volume thresholds exceeded, not just peak rates)

• Overall water cycle sustainability

Three Routes to Achieve Credit 2:

Route A: 30% Annual Volume Reduction

Demonstrate 30% reduction in annual run-off volume compared to pre-development.

Assessment methodology:

• Calculate total rainfall volume falling on site annually (typical UK: 600-1000mm/year depending on location)

• Determine pre-development percentage that became run-off (depends on permeability, vegetation)

• Calculate post-development run-off volume

• Demonstrate 30% reduction

Mechanisms achieving volume reduction:

• Infiltration: SuDS features allowing water to soak into ground rather than discharge

• Evapotranspiration: Green roofs, rain gardens, vegetated swales where plants absorb water

• Rainwater harvesting: Capturing roof water for toilet flushing, irrigation, vehicle washing

Typical approaches:

For small residential developments (houses with gardens):

• Permeable driveways infiltrating to ground

• Rain gardens in front/rear gardens

• Soakaways for roof water where ground conditions suitable

• Green roofs on garages/extensions

For medium commercial developments (offices, retail):

• Extensive green roofs across 40-60% of roof area

• Bioretention planters throughout site

• Permeable parking areas with infiltration

• Tree pits with structural soil and infiltration

For large urban developments (mixed-use, regeneration):

• Integrated blue-green infrastructure across public realm

• Podium-level rain gardens and green roofs

• Rainwater harvesting for toilet flushing in all buildings

• Substantial landscaping with absorbent soils

Modelling requirements:

• Annual simulation using continuous rainfall data (not just individual storm events)

• Modelling must account for:

  • Seasonal variation (winter soil saturation reduces infiltration)

  • Cumulative impacts (multiple storms with limited drying time between)

  • Feature capacity (green roofs saturate, reducing effectiveness)

  • Maintenance state (clogged infiltration reduces performance)

Evidence required:

• Annual water balance calculations

• Breakdown of volume reduction mechanisms

• Specifications for SuDS features providing volume reduction

• Infiltration test results (BRE Digest 365) if infiltration-based

• Green roof specifications with retention capacity modelling

• Rainwater harvesting system sizing and usage calculations

Route B: Maintain Pre-Development Infiltration Rates

Demonstrate that post-development infiltration to ground equals or exceeds pre-development infiltration. Suitable for greenfield sites with good infiltration characteristics.

Assessment approach:

• Determine pre-development infiltration rate through soil testing and calculations

• Design SuDS features providing equivalent infiltration capacity post-development

• Account for reduced pervious area due to buildings/hardstanding

• Demonstrate equivalent infiltration achieved through engineered SuDS

Typical scenarios:

• Greenfield site, naturally free-draining soils

• Post-development includes permeable paving, infiltration basins, soakaways

• Total infiltration capacity designed to match natural site infiltration

Challenges:

• Requires good ground conditions (infiltration rate ≥1×10⁻⁶ m/s minimum)

• Contaminated land may prevent infiltration (pollutant migration concerns)

• High groundwater prevents infiltration (requires >1m clearance)

• Clay soils very difficult to achieve equivalent infiltration

Evidence required:

• Infiltration testing across site (multiple test locations)

• Pre-development infiltration calculations

• Post-development infiltration design with feature sizing

• Demonstration that equivalent capacity provided

Route C: Rainwater Harvesting (50% Roof Capture)

Capture and reuse minimum 50% of rainfall from roof areas through rainwater harvesting systems.

System requirements:

• Storage tanks sized to capture 50% of annual roof run-off

• Non-potable water uses: toilet flushing, irrigation, vehicle washing, laundry (if commercial)

• System designed to BS EN 16941-1:2018 standards

• Mains top-up for periods when harvested water exhausted

• Overflow arrangements for tank capacity exceedance

Sizing methodology:

• Calculate annual rainfall on roof area (roof area × annual rainfall)

• 50% of this volume must be captured and reused

• Tank sizing using yield-after-spillage method accounting for:

  • Demand patterns (daily/weekly usage)

  • Rainfall patterns (seasonal variation)

  • First flush diversion (initial rainfall discarded for quality)

  • System efficiency losses

Typical tank sizes:

For residential house (100m² roof):

• Annual rainfall 700mm = 70,000 litres falls on roof

• 50% target = 35,000 litres captured

• Tank size typically 2,000-5,000 litres (sized for daily demand, not annual total)

For commercial office (2,000m² roof):

• Annual rainfall 600mm = 1,200,000 litres falls on roof

• 50% target = 600,000 litres captured

• Tank size typically 50,000-150,000 litres depending on toilet usage

Uses and demand:

• Toilet flushing typically 30-50 litres per person per day in offices

• Irrigation seasonal (high summer demand when rainfall lowest - challenging)

• Vehicle washing commercial sites (regular daily demand - good for consistent system use)

Challenges:

• Tank capital cost (£100-200 per m³ installed)

• Space requirements (substantial tank volumes needed)

• System complexity (pumps, controls, filtration, mains backup)

• Maintenance requirements (regular cleaning, filter replacement)

• Demand-supply mismatch (summer irrigation needs conflict with low rainfall)

Evidence required:

• Rainwater harvesting system design drawings

• Tank sizing calculations with yield-after-spillage methodology

• Demand calculations showing reuse arrangements

• Compliance with BS EN 16941-1:2018

• Maintenance schedule and responsibilities

Credit 2 Assessment:

Only one route needs achieving—not all three. Select most appropriate for site characteristics:

• Good ground conditions: Route B (infiltration) often most cost-effective

• Contaminated/high groundwater sites: Route C (rainwater harvesting) may be only viable option

• Mixed conditions: Route A (30% volume reduction) through combination of approaches

Assessor reviews evidence confirming one route achieved. Must demonstrate genuine volume reduction, not just theoretical calculations—post-construction verification increasingly required.

Credit 3: Water Quality Treatment (Pollution Prevention)

Objective: Prevent contaminated run-off polluting watercourses through appropriate treatment before discharge.

Requirements for Credit 3:

Treatment Train Approach Mandatory:

Version 7 requires minimum two-stage treatment for all developments (Version 6 allowed single-stage for low-risk sites):

Stage 1 - Primary Treatment:

• Sedimentation and gross solids removal

• Typical features: grass swales, filter strips, sedimentation basins

• Removes suspended solids, associated pollutants (heavy metals attach to sediments)

Stage 2 - Secondary Treatment:

• Filtration and biological treatment

• Typical features: bioretention cells, infiltration basins, filter drains

• Removes dissolved pollutants, oils, nutrients

Stage 3 - Tertiary Treatment (for high-risk sites):

• Enhanced removal of specific contaminants

• Typical features: proprietary treatment devices, petrol interceptors, constructed wetlands

• Required for high-risk areas (detailed below)

Version 7 Enhancement - Treatment Effectiveness Modelling:

Not sufficient to specify appropriate treatment features—must model and demonstrate treatment effectiveness using recognised methodologies.

CIRIA SuDS Manual (C753) Treatment Methodology:

Pollution hazard indices calculated for:

• Suspended Solids: Sediment-bound pollutants

• Heavy Metals: Zinc, copper, lead from vehicles/roofs

• Hydrocarbons: Oils from vehicles

• Nutrients: Nitrogen, phosphorus from landscaping/atmospheric deposition

Hazard determined by:

• Land use (residential low, commercial medium, industrial high)

• Traffic intensity (vehicles per day)

• Specific pollutant sources (petrol stations, vehicle washing, chemical storage)

Treatment features assigned mitigation indices showing pollutant removal capability. Design must demonstrate:

• Hazard indices for all pollutants calculated

• Treatment features provide adequate mitigation indices

• Overall treatment train achieves required pollutant removal

• Sensitive receiving waters (high ecological value) require enhanced treatment

Evidence requirement:

• CIRIA C753 calculations showing hazard and mitigation indices for each pollutant

• Demonstration that treatment train provides adequate mitigation

• Specifications for treatment features with pollutant removal capabilities

High-Risk Source Requirements:

For specific high-risk areas, enhanced treatment mandatory:

Vehicle Parking >20 Spaces or Commercial Vehicle Areas:

• Full retention Class 1 petrol interceptor (BS EN 858-1)

• Sized for 6-minute retention time at peak flow

• Bypass arrangement with high-level alarm preventing flooding but alerting to untreated discharge

• Shut-off valve preventing pollutant escape during spillages

Chemical/Fuel Storage Areas:

• Impermeable surfacing with sealed drainage

• Emergency containment (bunding to 110% of largest container)

• Automated shut-off valve preventing pollutants entering drainage

• Isolated drainage to foul sewer or specialist treatment

Vehicle Washing Areas:

• Full treatment system to EA Pollution Prevention Guidance (PPG13 successor guidance)

• Oil separation, silt removal, and potentially biological treatment

• Cannot discharge to surface water without EA permit

• Typically discharge to foul sewer after treatment

Industrial Processes:

• Specific treatment appropriate to pollutants generated

• May require bespoke treatment systems

• EA environmental permit likely required

• Discharge consent with quality limits

First Flush Management:

The first 5mm of rainfall carries disproportionate pollutant load (accumulated surface contaminants washed off). Version 7 requires:

• First flush captured and treated separately where feasible

• Treatment features sized for first 5mm across catchment area

• On very constrained sites where full capture impossible: justification and demonstration of maximum practical treatment

Post-Construction Verification (NEW Requirement):

Version 7 introduces mandatory post-construction water quality verification:

Before Final Certification:

• Water quality sampling at SuDS feature outlets

• Parameters: suspended solids, hydrocarbons, heavy metals (copper, zinc)

• Demonstrate treatment features functioning as designed

• Compared against design predictions from CIRIA C753 modelling

During Operational Phase:

• Annual water quality monitoring for first 3 years post-completion

• Monitoring results submitted to BREEAM assessor for post-construction review

• Evidence of maintenance activities informed by monitoring results

This represents significant change from Version 6, where post-construction assessment focused on as-built compliance rather than measured performance.

Monitoring Specification:

• Sample collection points at inlets and outlets of key treatment features

• Parameters align with CIRIA C753 pollutants of concern

• Sampling during/after storm events (not dry weather baseflow)

• Laboratory analysis to appropriate standards

• Reporting compared to design performance predictions

Evidence Requirements Credit 3:

Design Stage:

• CIRIA C753 hazard and mitigation calculations

• Treatment train description with two (or three) stages identified

• Feature specifications with pollutant removal capabilities

• For high-risk areas: petrol interceptor, shut-off valve, or treatment system specifications

Post-Construction:

• As-built drawings showing treatment features constructed as designed

• Commissioning records for treatment devices (interceptors, control valves)

• Initial water quality monitoring results

• Maintenance and monitoring schedule for ongoing performance verification

Operational Phase:

• Annual monitoring reports for years 1-3

• Evidence maintenance conducted (inspection records, cleaning logs, photographic evidence)

• Corrective actions taken if monitoring identifies performance issues

Mandatory Minimum Standards Version 7

Version 7 surface water credits become mandatory minimum requirements for higher BREEAM ratings.

For Excellent Rating:

Minimum 2 surface water credits required

This means achieving any two of:

• Credit 1: Run-off rate control (50% brownfield OR greenfield rates)

• Credit 2: Volume reduction (30% reduction OR infiltration OR 50% harvesting)

• Credit 3: Water quality treatment (two-stage train with monitoring)

Strategic Implications:

Most projects will target Credits 1 and 3 (rate control + water quality) as these align with planning/building regulations requirements anyway. Credit 2 (volume reduction) becomes the additional stretch target for 3 credits.

Alternatively, sites with excellent infiltration conditions might achieve Credits 2 and 3 (volume reduction + water quality) through comprehensive infiltration-based SuDS, with Credit 1 following automatically.

Cannot achieve Excellent without surface water credits—no trading with other categories to compensate.

For Outstanding Rating:

All 3 surface water credits mandatory

Outstanding requires:

• 50% brownfield run-off improvement (or greenfield rates)

• 30% volume reduction (or equivalent infiltration/harvesting)

• Two-stage treatment with post-construction verification

This represents substantial infrastructure and cost commitment. Outstanding developments must demonstrate exemplary surface water management through:

• Large-scale SuDS infrastructure (typically 25-40% of site area)

• Nature-based solutions visible and integrated with landscape

• Monitoring and maintenance regimes ensuring long-term performance

• Genuine environmental benefits beyond regulatory compliance

Combined with Flood Risk Minimums:

Outstanding also requires 2 flood risk credits (effectively Flood Zone 1 only). Combined with 3 surface water credits, this means:

Outstanding requires:

• Flood Zone 1 site (or exceptional resilience achieving 2 credits in Zone 2—very rare)

• 50% brownfield run-off improvement with climate change

• 30% volume reduction

• Two-stage water quality treatment with verification

• Post-construction monitoring for 3 years

This package typically adds £200,000-500,000 to development costs for medium-sized schemes (50-100 dwellings or equivalent commercial), representing 2-4% of construction budget.

Sites without space for extensive SuDS, or with ground conditions preventing infiltration, will struggle to achieve Outstanding under Version 7 regardless of performance in other categories.

SuDS Hierarchy and Justification Requirements

Version 7 strengthens emphasis on the SuDS hierarchy, requiring more robust justification for moving down the hierarchy away from infiltration and surface features.

SuDS Hierarchy (Preferred Order):

1. Infiltration at Source

Manage rainfall where it falls through infiltration to ground:

• Permeable paving

• Soakaways

• Rain gardens and bioretention

• Infiltration basins

2. Surface Attenuation Features

Visible surface water features providing storage and treatment:

• Swales and filter strips

• Detention basins and ponds

• Wetlands and reed beds

3. Underground Attenuation

Buried storage where surface features not viable:

• Cellular storage crates

• Oversized pipes

• Geocellular systems

4. Controlled Discharge to Watercourse

Direct discharge with flow control:

• Vortex flow controls

• Hydrobrakes

• Orifice plates

5. Controlled Discharge to Sewer

Discharge to combined or surface water sewer:

• Only where other options not feasible

• Requires sewerage undertaker agreement

• Subject to discharge limits and adoption standards

Version 7 Justification Requirements:

For each hierarchy level not achieved, must provide robust justification:

Cannot Infiltrate:

Valid justifications:

• Failed infiltration testing (rates <1×10⁻⁶ m/s confirmed through BRE Digest 365 tests)

• Contaminated land where infiltration risks pollutant migration

• High groundwater (less than 1m clearance from infiltration feature base)

• Clay/impermeable soils confirmed through site investigation

• Geological hazards (mining subsidence, karst features)

Invalid justifications:

• "Inconvenient" or "client preference"

• Cost saving (unless demonstrated infiltration genuinely unviable economically)

• Space constraints alone (must demonstrate space maximised for infiltration features)

Cannot Use Surface Features:

Valid justifications:

• Genuinely insufficient space (site coverage >80%, no landscaping areas)

• Heritage/conservation constraints preventing surface modifications

• Contaminated land requiring capping (surface features would breach cap)

• Security concerns for specific building types (data centres, defence establishments)

Invalid justifications:

• Aesthetic preference for underground solutions

• Maintenance concerns (surface features require maintenance but this isn't justification to avoid them)

• "Client doesn't want visible water features"

Evidence Requirements:

Justification must include:

• Site investigation reports confirming ground conditions

• Infiltration test results with locations shown on plans

• Contamination assessment if contaminated land claimed

• Groundwater monitoring data if high groundwater claimed

• Space analysis showing SuDS feature sizing attempted at maximum viable scale

• Professional certification from competent drainage consultant confirming justification validity

Partial Hierarchy Compliance:

Version 7 recognises many sites can partially comply with hierarchy:

• Infiltration viable in some areas, not others

• Surface features possible for portion of site

• Underground storage necessary for remainder

This is acceptable—maximise highest-hierarchy solutions across site, use lower-hierarchy solutions only where genuinely necessary. Document which areas achieve which hierarchy levels and justification for variations across site.

Version 7 Enhancement:

Assessors more rigorously scrutinise hierarchy justification than Version 6. Generic statements like "ground conditions unsuitable for infiltration" rejected without supporting SI evidence. Projects must demonstrate genuine attempts to maximise hierarchy compliance, not simply defaulting to conventional underground tanks because they're familiar or preferred by contractors.

Climate Change Allowances for Surface Water

Version 7 updates and clarifies climate change allowances for surface water drainage design.

Allowances Applied:

Peak Rainfall Intensity: +40% applied to post-development scenarios (most of England)

High-Risk Catchments: +45% in catchments identified as higher sensitivity

Critical Drainage Areas: +50% where local authorities designate special drainage management requirements

Application Methodology:

Climate change factors apply to post-development calculations only, not pre-development baselines:

Pre-development: Historical rainfall data and conditions

Post-development: Historical rainfall data × climate change factor

This is logical—assessing how future development performs under future climate conditions, compared to how site functioned historically under historical climate.

Calculation Example:

1-in-100 year, 60-minute storm:

Historical rainfall depth: 50mmWith +40% climate change: 70mm

Pre-development run-off calculation:50mm rainfall → calculate run-off from historic site conditions → determine peak discharge

Post-development run-off calculation:70mm rainfall → calculate run-off from developed site → design attenuation to achieve 50% improvement versus pre-development

Version 7 Clarifications:

Several common Version 6 confusions now explicitly addressed:

Climate change applies to rainfall intensity, not just peak flows:Some Version 6 projects applied climate change to river flooding (+40% flows) but not surface water drainage. Version 7 requires +40% rainfall intensity for surface water explicitly.

Both return periods (1-year and 100-year) include climate change:Apply +40% to both. Some projects applied climate change to 100-year only—Version 7 confirms both require allowances.

Allowances cover building lifetime (60 years minimum):Project through 2080s scenarios minimum (60 years from 2025 completion). Critical infrastructure requires 120-year projections (2140s scenarios).

Regional Variations:

Scotland: +30% to +50% depending on catchment characteristics (SEPA guidance)

Wales: +40% to +60% with regional variation (NRW guidance)

Northern England upland catchments: +50% reflecting enhanced sensitivity

Urban catchments with rapid response: +50% due to flashy hydrology

Evidence Requirements:

Climate change documentation must include:

• Confirmation of allowance percentage applied (+40%, +45%, or +50%)

• Reference to EA/SEPA/NRW guidance justifying allowance selected

• Justification if lower allowances used (requires robust reasoning)

• Demonstration allowances applied to both 1-year and 100-year events

• Confirmation applied to rainfall intensity, not just peak flows

How to Achieve Version 7 Surface Water Credits

Achieving all three Version 7 surface water credits requires integrated drainage strategy from project inception, substantial SuDS infrastructure, and commitment to long-term monitoring and maintenance.

Stage 1: Feasibility and Site Appraisal

Ground Conditions Investigation:

Commission preliminary site investigation at feasibility stage:

• Trial pits or boreholes across site

• Infiltration testing to BRE Digest 365 standard

• Groundwater monitoring (ideally 12 months, minimum 6 months)

• Contamination screening

This £5,000-15,000 investment determines drainage strategy viability:

• Good infiltration (>1×10⁻⁵ m/s): Credit 2 achievable through infiltration route, large attenuation volumes avoided

• Moderate infiltration (1×10⁻⁶ to 1×10⁻⁵ m/s): Partial infiltration viable, supplemented with surface attenuation

• Poor infiltration (<1×10⁻⁶ m/s): Credit 2 requires rainwater harvesting or challenging volume reduction through evapotranspiration only

• Contaminated ground: Infiltration not viable, sealed drainage required, Credit 2 very difficult

Ground investigation findings should inform land acquisition decisions. Sites with excellent infiltration conditions are substantially more valuable under Version 7 than sites requiring sealed drainage.

Space Allocation:

Allocate indicative space for SuDS during feasibility masterplanning:

For 50% brownfield improvement:

• Surface SuDS features: 8-15% of site area typically required

• OR underground storage: 2-4% of site area (though works against hierarchy)

For 30% volume reduction:

• Infiltration basins: additional 5-10% of site area

• OR rainwater harvesting: space for tanks (typically underground or under buildings)

• OR green roofs: 40-60% of roof area

For two-stage treatment:

• Surface features provide treatment whilst performing other functions (count once)

• High-risk areas: isolated drainage and treatment (1-2% additional site area)

Total SuDS allocation: 15-25% of site area for surface-based approaches achieving all 3 credits.

This compares to 5-10% typical under Version 6 requirements—substantial increase in land dedicated to drainage infrastructure.

Preliminary Cost Estimation:

Surface water infrastructure costs increase significantly under Version 7:

Version 6 typical costs (30% improvement, basic treatment):£80,000-150,000 for 50-dwelling residential scheme

Version 7 typical costs (50% improvement, volume reduction, enhanced treatment):£120,000-250,000 for equivalent scheme

Cost breakdown Version 7:

• Attenuation storage (50% larger): +£30,000-60,000

• Volume reduction features (infiltration/harvesting): +£20,000-50,000

• Enhanced treatment train: +£10,000-20,000

• Monitoring equipment and 3-year monitoring: +£10,000-15,000

These costs should be incorporated in feasibility appraisals, representing 1.5-2.5% of construction budget typically.

Stage 2: Concept Design (RIBA Stage 2)

Engage Drainage Consultant:

Appoint suitably qualified drainage consultant during concept design with competencies BREEAM specifies:

• Capability to champion SuDS within design team

• Knowledge of infiltration-based and surface drainage solutions

• Experience with CIRIA C753 methodology

• Hydraulic modelling capability (MicroDrainage, InfoWorks, or equivalent)

• Understanding of water quality treatment design

Professional qualifications typically:

• Chartered engineer (ICE, CIWEM) with drainage specialism

• OR landscape architect with specialist SuDS training and hydraulic modelling capability

• Demonstrable Version 7 BREEAM experience

Integrated Landscape and Drainage Strategy:

SuDS features should integrate with landscape architecture from concept stage:

Swales: Designed as attractive green corridors, planted with native species, providing pedestrian routes alongside drainage function

Rain gardens: Designed as ornamental planted features in courtyards, squares, building entrances—visible amenity whilst managing water

Detention basins: Designed as landscape ponds or meadows, providing public amenity and biodiversity alongside storage

Permeable paving: Specified for parking, courtyards, shared surfaces—contributes to place-making whilst managing infiltration

Best SuDS schemes result from landscape architect and drainage engineer working collaboratively from concept stage, not drainage engineer retrofitting technical solutions into completed landscape design.

Preliminary Drainage Strategy:

Develop concept-stage drainage strategy addressing:

Credit 1 - Run-off Rate Control:

• Calculate pre-development run-off rates (baseline establishment)

• Determine post-development run-off with climate change

• Identify 50% improvement target discharge rates

• Preliminary attenuation volume sizing

• Identify potential attenuation feature locations

Credit 2 - Volume Reduction:

• Select most appropriate route (infiltration, harvesting, or 30% reduction)

• If infiltration: preliminary feature sizing and locations based on SI results

• If harvesting: demand calculations and tank sizing

• If 30% reduction: identify mechanisms and preliminary modelling

Credit 3 - Water Quality:

• Identify pollution sources and hazard levels

• Design treatment train (minimum two stages)

• Preliminary feature specifications

• High-risk area identification and isolated drainage strategy

This preliminary strategy (£10,000-20,000) provides sufficient detail for planning submission and informs design development, preventing expensive revisions during technical design.

Stage 3: Developed Design (RIBA Stage 3)

Detailed Hydraulic Design:

Develop comprehensive hydraulic design using professional software:

MicroDrainage or InfoWorks ICM modelling:

• Detailed network model with all catchments, pipes, features

• Pre-development and post-development scenarios

• 1-year and 100-year events with climate change (+40%)

• Storage feature sizing with routing calculations

• Discharge control sizing and specification

Model outputs required:

• Flow-time diagrams showing attenuation functioning

• Storage volume calculations for all features

• Discharge rates at each outfall demonstrating 50% improvement

• Sensitivity analysis (blocked outlets, maintenance states, different storms)

• Long-duration storm assessment (6-hour, 12-hour events)

Credit 1 Evidence Package:

• Pre and post-development run-off calculations

• Attenuation storage calculations

• Discharge control specifications (hydrobrake curves, orifice sizing)

• Exceedance routing showing safe pathways for flows exceeding design capacity

Volume Reduction Design:

If Infiltration Route:

• Infiltration feature sizing (soakaways, basins, permeable paving)

• Structural specification (depths, materials, geotextiles)

• Positioning ensuring 1m clearance from groundwater

• Pre-treatment before infiltration (preventing clogging)

• Maintenance access and inspection arrangements

If Rainwater Harvesting Route:

• Tank sizing using yield-after-spillage methodology

• Demand calculations (toilet flushing, irrigation, other uses)

• System schematic (tanks, pumps, filters, controls, mains backup)

• BS EN 16941-1 compliance demonstration

• Maintenance and inspection schedule

If 30% Volume Reduction Route:

• Annual water balance modelling using continuous rainfall data

• Breakdown of mechanisms (green roofs, bioretention, infiltration, harvesting)

• Each mechanism quantified contribution

• Combined effectiveness demonstrating 30% reduction

Credit 2 Evidence Package:

• Volume reduction calculations with methodology clearly shown

• Feature specifications providing volume reduction

• Infiltration or harvesting system design

• Demonstration chosen route achieved

Water Quality Treatment Design:

CIRIA C753 Methodology:

• Pollution hazard index calculations for suspended solids, metals, hydrocarbons, nutrients

• Treatment mitigation indices for each SuDS feature

• Demonstration treatment train provides adequate mitigation overall

Treatment Feature Specifications:

Stage 1 - Primary Treatment:

• Swale dimensions (base width, side slopes, depth, length)

• Or filter strip specifications (width, vegetation type)

• Designed for flow velocities <0.5 m/s (promotes sedimentation)

Stage 2 - Secondary Treatment:

• Bioretention cell specifications (dimensions, filter media layers, plant species)

• Or infiltration basin design (surface area, depth, inlet/outlet arrangements)

• Underdrainage if infiltration limited

Stage 3 - Tertiary Treatment (high-risk areas only):

• Petrol interceptor sizing and specification (BS EN 858-1)

• Shut-off valve specifications with alarm arrangements

• Isolated drainage from high-risk areas

Monitoring Design:

• Sample point locations (inlets and outlets of treatment features)

• Access arrangements for sampling

• Parameters to be monitored (suspended solids, hydrocarbons, heavy metals)

• Frequency (quarterly year 1, biannually years 2-3)

Credit 3 Evidence Package:

• CIRIA C753 hazard and mitigation calculations

• Treatment train description with two/three stages identified

• Feature specifications with pollutant removal capabilities

• High-risk area treatment (interceptors, shut-off valves)

• Monitoring strategy for post-construction verification

Maintenance Strategy:

Version 7 requires detailed maintenance strategy for all SuDS features:

Feature-by-Feature Maintenance Schedule:

Permeable paving:

• Frequency: Annual vacuum sweeping

• Tasks: Remove sediment, check infiltration rates, replace aggregate if performance degraded

• Responsibility: Management company or highway authority

• Cost: £2-5 per m² annually

Swales and filter strips:

• Frequency: Quarterly inspection, annual maintenance

• Tasks: Vegetation management, sediment removal, erosion repair

• Responsibility: Landscaping contractor under management company

• Cost: £5-10 per linear metre annually

Bioretention and rain gardens:

• Frequency: Quarterly inspection, seasonal maintenance

• Tasks: Plant management, mulch replenishment, sediment removal, filter media replacement (5-10 year cycle)

• Responsibility: Specialist SuDS maintenance contractor

• Cost: £20-40 per m² annually

Attenuation basins and ponds:

• Frequency: Quarterly inspection, annual maintenance

• Tasks: Vegetation management, sediment removal (5-10 year cycle), structural inspection

• Responsibility: Management company or adopted by local authority

• Cost: £10-20 per m² annually

Petrol interceptors:

• Frequency: Quarterly emptying minimum

• Tasks: Sludge and oil removal, bypass valve testing, alarm verification

• Responsibility: Specialist contractor under building management

• Cost: £500-1,000 per visit

Rainwater harvesting:

• Frequency: Quarterly filter inspection, annual system service

• Tasks: Filter replacement, tank cleaning, pump maintenance, control testing

• Responsibility: Building management

• Cost: £1,000-2,000 annually for commercial systems

Maintenance Agreement Requirements:

Version 7 requires maintenance agreements demonstrating:

• All features covered with defined maintenance tasks and frequencies

• Responsible parties identified and agreed

• Funding arrangements in place (service charges, adoption with commuted sums, management company budgets)

• Inspection and reporting procedures

• Corrective action procedures if monitoring identifies performance issues

Agreements must be legally complete before final BREEAM assessment—draft agreements insufficient.

Stage 4: Technical Design and Pre-Construction

Final Calculations and Specifications:

Complete detailed calculations supporting BREEAM submission:

Run-off Control:

• Final hydraulic model outputs with all catchments and features

• Sensitivity analysis confirming performance across range of scenarios

• Discharge consents from EA/IDB or sewerage undertaker agreements

Volume Reduction:

• Annual water balance modelling outputs

• Volume reduction mechanism specifications

• Infiltration test results or rainwater harvesting system detailed design

Water Quality:

• Final CIRIA C753 calculations

• Treatment feature specifications with manufacturer data

• High-risk area treatment system details

Construction Specifications:

Detailed specifications ensuring correct installation:

Infiltration features:

• Excavation depths and side slopes

• Geotextile specifications and installation methods

• Aggregate specifications (size, cleanliness, permeability)

• Inspection and testing requirements during construction

Treatment features:

• Filter media specifications (particle size distribution, permeability, organic content)

• Plant species and planting densities

• Inlet and outlet structures with levels

• Erosion protection at inflows

Control structures:

• Hydrobrake or vortex valve specifications with performance curves

• Orifice plate sizing and materials

• Access chamber specifications for inspection and maintenance

Site Establishment and Construction:

During construction phase:

Protecting infiltration features:

• Delineate SuDS feature locations, prevent trafficking

• Install sediment controls preventing construction run-off entering features

• Construct SuDS features last (after building completion reduces silt loading)

Quality control:

• Infiltration testing post-construction confirming performance not degraded

• As-built surveys confirming levels achieved as designed

• Photographic records of construction stages

Commissioning:

• Flow testing confirming discharge rates achieve design specifications

• Control structure testing under multiple flow conditions

• Rainwater harvesting system commissioning (if applicable)

Stage 5: Post-Construction and Operational Monitoring

Initial Performance Verification:

Before BREEAM final assessment:

As-Built Evidence:

• Drawings showing SuDS features constructed as designed (or variations documented and approved)

• Survey confirming levels, volumes, and dimensions

• Material specifications confirming compliant products used

Initial Water Quality Monitoring:

• Sample collection from SuDS feature outlets during/after storm events

• Analysis for suspended solids, hydrocarbons, heavy metals (copper, zinc)

• Comparison with design predictions from CIRIA C753 modelling

• Report demonstrating treatment features functioning

Maintenance Arrangements:

• Management company established with SuDS maintenance in scope

• Maintenance contracts in place with contractors

• OR adoption agreements completed with commuted sums transferred

• OR for residential, homeowner guidance prepared and issued

BREEAM Submission:

• Complete evidence package with calculations, specifications, as-built drawings

• Initial monitoring results

• Maintenance agreements

• Photographic evidence of completed features

Operational Monitoring (Years 1-3):

Version 7 requires ongoing monitoring demonstrating sustained performance:

Year 1 - Quarterly Monitoring:

• Water quality sampling four times during year 1

• Quarterly inspections of all features

• Maintenance activities logged (sediment removal, vegetation management)

• Report to BREEAM assessor end of year 1

Years 2-3 - Biannual Monitoring:

• Water quality sampling twice per year

• Biannual inspections

• Continued maintenance with logging

• Annual reports to BREEAM assessor

Monitoring Parameters:

• Suspended solids (mg/l)

• Total petroleum hydrocarbons (mg/l)

• Heavy metals: copper, zinc, lead (μg/l)

• Additional parameters if specific pollutants identified (nutrients for agricultural catchments, etc.)

Corrective Actions:

• If monitoring identifies underperformance: investigation required

• Remedial measures implemented (additional maintenance, feature modifications)

• Further monitoring confirming performance recovered

Cost Implications:

3-year monitoring programme typically costs:

• Water quality analysis: £500-1,000 per sampling event

• Year 1 (four events): £2,000-4,000

• Years 2-3 (two events each): £1,000-2,000 per year

• Inspection and reporting: £1,000-2,000 per year

• Total 3-year monitoring: £6,000-12,000

These costs should be built into project budgets and management company service charges from outset.

Common Challenges with Version 7 Surface Water

Challenge 1: Achieving 50% Brownfield Improvement on Heavily Surfaced Sites

Sites with extensive historic hardstanding (100% impermeable car parks, industrial yards) face severe challenges achieving 50% improvement.

The Mathematics:

Historic site: 100% impermeable, Qbar = 10 l/sPost-development target: 5 l/s maximum (50% reduction)Post-development with buildings (60% impermeable): Natural run-off would be 6 l/sRequired attenuation: Reduce 6 l/s down to 5 l/s (marginal reduction)

But with climate change (+40% rainfall):Post-development with climate change: 8.4 l/sRequired attenuation: Reduce 8.4 l/s down to 5 l/s (40% reduction from post-development)

This requires substantial attenuation storage—often 40-60m³ per hectare of development, more if infiltration not viable.

Solutions:

Option A - Maximise Infiltration:

• Even if full infiltration not viable, partial infiltration reduces attenuation volumes

• Permeable paving across parking areas (even if only 30-40% infiltration efficiency)

• Bioretention features throughout site (every 5-10 parking spaces)

Option B - Green Infrastructure:

• Extensive green roofs (capture rainfall before becomes run-off)

• Podium gardens and tree pits with structural soil

• Reduces post-development run-off generation, less attenuation required

Option C - Very Large Underground Storage:

• If surface/infiltration options exhausted, resort to large underground tanks

• Works against SuDS hierarchy but may be only viable solution on constrained urban sites

• Requires robust justification for BREEAM assessor

Option D - Off-Site Mitigation:

• Occasionally viable to provide SuDS features off-site (adjacent land, highway verges)

• Requires agreements with landowners and statutory bodies

• BREEAM assessor discretion whether acceptable

Challenge 2: Volume Reduction on Contaminated Sites

Contaminated brownfield sites cannot infiltrate (pollutant migration risk), making Credit 2 (volume reduction) very difficult to achieve.

Infiltration Route - Blocked: Cannot infiltrate on contaminated land without full remediation (often uneconomic).

30% Volume Reduction - Challenging: Without infiltration, volume reduction relies on:

• Evapotranspiration from green roofs and rain gardens (with sealed bases)

• Rainwater harvesting

Evapotranspiration alone rarely achieves 30% volume reduction in UK climate (limited by solar radiation and temperature).

Rainwater Harvesting Route: Becomes the viable option but creates challenges:

For residential developments:

• Individual house tanks (2,000-5,000 litres each)

• Communal systems complex for management

• Cost: £2,000-5,000 per dwelling

For commercial developments:

• Large central tanks (50,000-150,000 litres)

• Space requirements (often in basement or under car park)

• Cost: £50,000-150,000 for medium office

Solutions:

Option A - Full Site Remediation:

• If development value justifies it, remediate to enable infiltration

• Typically requires removing contaminated soils or capping with clean materials

• Cost: £50-200 per m² depending on contamination level

Option B - Partial Remediation:

• Remediate clean areas of site enabling infiltration there

• Sealed drainage from contaminated areas

• Hybrid approach maximising infiltration where viable

Option C - Extensive Rainwater Harvesting:

• Design comprehensive harvesting system capturing 50%+ of roof run-off

• Accept cost premium as necessary for Version 7 compliance on contaminated sites

Option D - Accept 2 Credits Maximum:

• If Credit 2 uneconomical, accept maximum 2 surface water credits

• Achieves Excellent rating requirement (2 credits minimum)

• Outstanding unachievable but this may be acceptable for site characteristics

Challenge 3: High Groundwater Preventing Infiltration

Sites with high groundwater (less than 1m below proposed infiltration features) cannot achieve effective infiltration, blocking volume reduction route.

The Problem:

Infiltration requires minimum 1m clearance between infiltration feature base and maximum groundwater level. This ensures:

• Adequate unsaturated zone for water to percolate through

• Prevents infiltration features becoming submerged when groundwater rises

• Reduces risk of groundwater contamination

Sites in chalk, gravel, or low-lying areas often have groundwater <1m depth.

Assessment:

Groundwater monitoring through site investigation over 6-12 months determines:

• Typical groundwater levels

• Seasonal variation (winter highs, summer lows)

• Maximum observed levels

If maximum groundwater is <1m below ground surface, infiltration-based SuDS not viable.

Solutions:

Option A - Raised SuDS Features:

• Elevate entire development on platform to create 1m clearance

• SuDS features constructed on platform with infiltration into ground below

• Only viable if space for earthworks and retaining walls

Option B - Lined Features with Underdrainage:

• Construct bioretention/rain gardens with impermeable liners

• Underdrainage to attenuation storage

• Provides treatment and evapotranspiration but not infiltration volume reduction

• Achieves Credit 3 (treatment) but not Credit 2 through infiltration route

Option C - Rainwater Harvesting Route:

• Switch to 50% roof run-off harvesting for Credit 2

• Lined surface features for Credit 3 treatment

• Attenuation storage for Credit 1 run-off rate control

Option D - Evapotranspiration Focus:

• Extensive green roofs (50-70% coverage)

• Lined rain gardens with high evapotranspiration plants

• Achieve 30% volume reduction through evapotranspiration, not infiltration

• Challenging in UK climate but possible with extensive green infrastructure

High groundwater sites should prioritise rainwater harvesting route for Credit 2—most reliable pathway when infiltration blocked.

Challenge 4: Maintenance Agreement Timing

Version 7 requires maintenance agreements in place before final assessment, but developments often have complex adoption arrangements taking months to finalise.

The Problem:

Residential Developments:

• Mix of private gardens (homeowner maintenance), communal areas (management company), and adopted highways/SuDS (local authority)

• Management company may not be established until properties sold

• Local authority adoption agreements take 6-12 months to negotiate

Commercial Developments:

• Speculative developments where eventual occupier unknown at completion

• Adoption by water company or local authority requires commuted sums and legal agreements

• Facilities management contracts may not be in place at practical completion

BREEAM Requirement: Final assessment cannot be completed until maintenance agreements legally complete, not just in draft.

Solutions:

Option A - Management Company Pre-Establishment:

• Establish management company during construction phase

• First residents become members immediately on occupation

• SuDS maintenance written into management company articles from outset

Option B - Developer Maintenance Period:

• Developer retains maintenance responsibility for initial period (1-2 years)

• Management company or adoption arrangements finalised during this period

• BREEAM final assessment conducted once arrangements complete

Option C - Adoption Agreement Acceleration:

• Engage with local authority/water company early (concept design stage)

• Agree adoption in principle subject to construction to adoptable standards

• Fast-track legal agreements during construction

• Complete before practical completion enabling immediate final assessment

Option D - Homeowner Guidance for Private Areas:

• For private gardens with SuDS features (permeable driveways, soakaways)

• Provide comprehensive homeowner guidance with property conveyance

• Guidance explains features, maintenance requirements, and importance

• Counts as maintenance arrangement for BREEAM purposes if comprehensive and issued to all homeowners

Timing Strategy:

Begin maintenance arrangement discussions 12 months before practical completion. This allows:

• 3-6 months initial negotiations

• 3-6 months legal agreement drafting

• 1-3 months committee approvals (local authorities often require cabinet approval)

• Completion at or shortly after practical completion

Leaving this to post-completion creates 6-12 month delay in BREEAM final certification.

Version 7 Surface Water FAQ

Our Version 6 project achieved credits with 30% brownfield improvement. How much will upgrading to 50% for Version 7 cost?

Typically, increasing from 30% to 50% improvement adds 40-60% to surface water infrastructure costs. For a 50-dwelling residential scheme:

Version 6 (30% improvement): £100,000-150,000Version 7 (50% improvement): £140,000-240,000Additional cost: £40,000-90,000

The increase reflects:

• Larger attenuation volumes (50-80% more storage)

• More extensive SuDS features (additional area)

• Enhanced treatment requirements

However, this assumes similar approach (surface-based SuDS). If switching from underground tanks (Version 6) to comprehensive surface features (Version 7), costs may increase more substantially due to landscaping integration.

Can we achieve 50% improvement through underground tanks, or must we use surface SuDS?

Version 7 doesn't mandate surface features, but the SuDS hierarchy requires justifying why surface features not viable. Underground tanks alone will struggle to achieve all three credits:

Credit 1 (rate control): Achievable with tanksCredit 2 (volume reduction): Very difficult with tanks alone (no infiltration, no evapotranspiration)Credit 3 (water quality): Tanks provide settlement but not full two-stage treatment

Most projects need combining underground storage with surface treatment features to achieve all three credits.

If genuinely insufficient space for surface features (site coverage >85%, heritage constraints, contaminated land capping), underground approach with justification may achieve 2 credits (sufficient for Excellent), but Outstanding (requires 3 credits) becomes very difficult.

Does rainwater harvesting count towards both run-off reduction AND volume reduction credits?

Partially. Rainwater harvesting affects both but must meet different criteria:

For Credit 1 (rate control):

• Harvesting reduces run-off rates by capturing roof water

• However, must still demonstrate 50% improvement at all discharge points

• When harvesting tank full and overflowing, system provides no rate control

• Hydraulic calculations must assume tank unavailable (worst case) for storm routing

For Credit 2 (volume reduction):

• Harvesting capturing and reusing 50% of roof run-off achieves Credit 2

• Directly counts towards volume reduction

So harvesting contributes to both credits, but must be sized conservatively for Credit 1 (assume unavailable) whilst sized optimistically for Credit 2 (assume functioning).

Our site is greenfield—do we need 50% improvement or just maintain greenfield rates?

Greenfield sites must maintain greenfield run-off rates (not 50% improvement). However, several Version 7 enhancements apply:

Updated IH124 methodology: Use 2024 version calculating greenfield rates, not old methods

Climate change applied asymmetrically: Post-development includes +40% rainfall, pre-development does not

Both return periods: 1-year and 100-year events both must achieve greenfield rates

In practice, this requires substantial attenuation on most greenfield sites—typically 40-80m³ per hectare. The 50% improvement doesn't apply, but climate change application means significant infrastructure still needed.

How do we model evapotranspiration from green roofs for volume reduction credit?

Evapotranspiration modelling requires continuous simulation (not individual storm events) using:

Methodology:

• Continuous rainfall record for site location (10+ years)

• Green roof water balance model accounting for:

  • Rainfall onto roof

  • Evapotranspiration from substrate and plants (depends on temperature, solar radiation, wind)

  • Storage within substrate (depends on depth and saturation state)

  • Overflow when substrate saturated

Software:

• Specialist green roof modelling tools (GreenRoof Designer, GrowGreen)

• OR hydrological models with soil moisture accounting (InfoWorks ICM, SWMM)

Typical Performance:

Extensive green roof (80-150mm substrate):

• 30-50% annual run-off reduction (UK climate)

• Higher in summer (more evapotranspiration), lower in winter

Intensive green roof (>300mm substrate):

• 50-70% annual run-off reduction

• Greater storage capacity and more substantial vegetation

For Credit 2, green roofs contribute to overall 30% volume reduction across entire site, not 30% from roofs alone. Combine with other features (bioretention, infiltration) to achieve overall target.

Get Expert Version 7 Surface Water Support

Version 7 surface water requirements represent step-change in complexity, infrastructure requirements, and long-term commitment compared to Version 6. Achieving all three credits for Outstanding rating requires sophisticated drainage design, substantial SuDS infrastructure, and verified operational performance.

Our CIWEM-qualified drainage consultants provide comprehensive SuDS design services aligned with Version 7 standards:

Feasibility Stage Services:

• Site investigation coordination (infiltration testing, groundwater monitoring)

• Preliminary drainage strategy options appraisal

• 50% improvement feasibility assessment

• Budget cost estimates for Version 7 compliance

Design Stage Services:

• Detailed hydraulic modelling (MicroDrainage, InfoWorks ICM)

• 50% brownfield run-off improvement design

• Volume reduction strategy (infiltration, harvesting, or evapotranspiration)

• CIRIA C753 water quality treatment design

• SuDS hierarchy justification where surface features constrained

• Integration with landscape architecture and public realm design

BREEAM Certification Support:

• Evidence package preparation covering all three credits

• CIRIA C753 hazard and mitigation calculations

• Post-construction monitoring design

• Maintenance strategy and agreements coordination

Post-Construction Services:

• As-built verification and commissioning support

• Initial water quality monitoring programme

• 3-year operational monitoring coordination

• Corrective action recommendations if performance issues identified

Version 6 to Version 7 Transition:

• Gap analysis identifying additional requirements

• Cost implications of upgrading from 30% to 50% improvement

• Credit achievement feasibility (can all 3 credits be achieved?)

• Strategic recommendation on version selection

Based in London and operating throughout Kent, Essex and Scotland, we deliver integrated flood risk and surface water management services ensuring coordinated climate change assumptions and complementary strategies.

Our team combines hydrological expertise with practical construction experience and landscape integration capability. We work alongside architects and landscape architects from concept stage, ensuring SuDS becomes attractive, functional landscape infrastructure rather than engineering afterthought.

Professional indemnity insurance exceeds £5 million, and we maintain robust quality assurance procedures meeting Version 7 evidence standards. We've supported multiple Version 7 projects achieving all three surface water credits through innovative SuDS design on constrained urban sites.

Contact us to discuss your Version 7 surface water requirements