How to Discharge a Surface Water Drainage Condition: What the Council's Flood Officer Wants to See

To discharge a surface water drainage condition, the applicant submits a drainage strategy that the council's flood officer can check line by line. It sets out the existing drainage, a proven discharge point, a discharge rate and a storage volume with calculations, a layout drawing with levels, an exceedance route and a maintenance plan. Each item is cross-referenced to the wording of the condition. This guide sets out what that submission contains and where applications stall, using a decided Forest of Dean application, a dental surgery extension in Lydney (P1568/25/FUL), as the worked example.

A drainage condition on your permission? Send the decision notice through our fee proposal form and we will set out what the condition needs and what it will take to discharge it. Our SuDS strategy service covers the strategy, the calculations and the drawings.

What a surface water drainage condition is, and why work cannot start before it is discharged

A drainage condition defers the detailed drainage design from the planning application to a later approval. Most are written as pre-commencement conditions, beginning "No development shall commence until...". The Planning Practice Guidance on the use of planning conditions is direct about the consequence: "Development carried out without having complied with a pre-commencement condition would be unlawful" and "may be the subject of enforcement action" (paragraph 007).

Many drainage conditions have a second limb that bites later, a pre-occupation requirement to install the approved system before the building is first used and to maintain it thereafter. The approval and the installation are separate obligations, and both are checked.

The details are submitted to the local planning authority as an application for approval of details reserved by condition, with a fee. The same guidance gives the authority 8 weeks to decide, or a longer period agreed in writing, and requires the fee to be returned if no decision is made within 12 weeks (paragraph 034). Deemed discharge may be available for some conditions after a notice served at least 6 weeks after the application was received (paragraph 043). In practice the council's flood officer, or the Lead Local Flood Authority on major schemes, is consulted and the case officer follows that advice. A submission the flood officer cannot check is returned with questions, and the decision waits for the revised details.

In the Lydney case, condition 10 of the decision notice of 18 August 2026 reads:

"No development shall commence until proposals for foul and surface water drainage with sufficient details incorporating where appropriate the principle of Sustainable Drainage Systems (SUDS) within the development have been submitted to and approved in writing by the Local Planning Authority. Prior to the first use of the dental practice, the approved drainage shall be carried out in accordance with the approved details and permanently maintained as such thereafter."

The stated reason is that insufficient information had been submitted with the application to demonstrate satisfactory drainage, with reference to Policy CSP.2 of the Forest of Dean Core Strategy. The condition covers foul drainage as well as surface water.

Read the condition and the officer's comments first

A drainage condition is rarely detailed. The detail sits in the consultation responses on the planning file, and those responses are what the flood officer will check the submission against. The first task is to read every flood officer comment on the application and turn each requirement into a line of a compliance table.

The Lydney history shows why. The Forest of Dean Flood Risk Management Officer (FRMO) responded on 5 February 2026 that "Evidence of the existing/proposed surface water drainage system is required. This can be met via pre-commencement condition if necessary." The same response set out the requirements: surface water contained on site up to the 1 in 100-year event plus 40% for climate change; confirmation and evidence of the existing drainage; the age, size and location of any existing soakaway, with rubble-filled soakaways over 10 years old to be rejuvenated or replaced; and, "If infiltration is not viable", attenuation "prior to controlled discharge in line with the QBAR greenfield runoff rate."

AFC then prepared the planning-stage SuDS strategy, Issue 01, dated 18 June 2026. The officer report summarises the FRMO's comments of 3 July 2026 on AFC's planning-stage strategy: the strategy was "not acceptable in its current form", because "no site-specific infiltration testing has been provided, the proposed discharge destination remains unclear, existing drainage arrangements have not been confirmed". AFC issued Issue 02 on 5 July 2026, based on about 180 m² of paving over a 300 mm sub-base with partial infiltration assumed. On 7 August 2026, after the agent asked for a condition, the FRMO agreed: "on the basis that a public combined sewer exists in the proximity it can be safely assumed a drainage destination is available. As such we can condition the required information." Permission followed on 18 August 2026.

Every one of those points comes back at the condition stage. The condition-stage strategy that AFC prepared in October 2026 is a revised design, not a resubmission of Issues 01 and 02. It opens with two tables: one setting out where each requirement of the condition is addressed (Figure 1), and one recording each FRMO request with the response (Figure 2).

Figure 1: Answer the condition line by line. Condition requirements table, redrawn from the Lydney strategy (Table 1, Issue 03, 27 October 2026).

Figure 2: Every request on the planning file gets a response, including the ones the design does not follow, with the reason.

Step 1: confirm the existing drainage

The flood officer needs to know what the site does now, because the existing arrangement sets the baseline for the discharge rate and shows where the new drainage can connect. The minimum is the water company's sewer records, the existing manholes and downpipes, and a statement of what drains where. A CCTV drainage survey is the firmer evidence.

Levels come first: they decide where water goes. The Environment Agency's 1 m LiDAR terrain model gave approximately 24.5 mAOD at the front of the Lydney building, 23.9 mAOD at its rear wall and 23.3 mAOD at the east corner of the rear yard, a fall of about 0.6 m over 12 m (Figure 3). No measured survey was available when the strategy was written, so the levels were stated as assumed from LiDAR.

Figure 3: Levels first: they decide where water goes. Environment Agency LiDAR 1 m terrain model around the site.

The Severn Trent wastewater asset plan (obtained 19 October 2026) showed no public surface water sewer in Victoria Road. It showed a public combined sewer whose head manhole, MH 2001, lies about 11 m from the site (cover level 23.32 mAOD, invert level 21.31 mAOD), and a public foul lateral drain at MH 2007 in the neighbouring plot. The existing roof downpipes were assumed, from the architect's drawings and client information, to discharge into the private foul drain that runs to MH 2007. That is surface water entering a drain the water company records as foul.

The strategy recorded this on drawing 112-DR-01 as a desk-based record (Figure 4), stated which parts were assumed, and set out what the CCTV survey before construction must establish. A desk record with its assumptions stated is acceptable evidence where a survey cannot yet be done. A drawing that shows assumed routes as fact is not.

Figure 4: Confirm what is there before you design. Existing drainage, desk-based record (extract from drawing 112-DR-01 P6).

Step 2: prove the discharge point by the drainage hierarchy

The Flood risk and coastal change Planning Practice Guidance (paragraph 056, Reference ID 7-056-20220825) sets the order in which discharge destinations are to be considered: "into the ground (infiltration); to a surface water body; to a surface water sewer, highway drain, or another drainage system; to a combined sewer." The Defra National Standards for Sustainable Drainage Systems (2025) put collection for non-potable use above infiltration (clause 1.2), and clause 1.3 adds that "Higher cost alone shall not be a reason to utilise lower priority final destinations." Since the August 2026 NPPF, policy F8 asks for SuDS designed in accordance with those standards; the change is set out in the AFC article on SuDS under the NPPF 2026.

A strategy has to show each level tested or ruled out with evidence, in order (Figure 5).

Figure 5: The drainage hierarchy, with the evidence each level needs. Drawn from PPG paragraph 056, clause 1.2 of the National Standards and the Lydney strategy's Table 4.

Collection for non-potable use. The only non-potable demand at Lydney is WC flushing for two WCs, and storage for re-use would not reliably be empty at the start of a storm, so it is not counted as attenuation.

Infiltration. Infiltration is accepted on site-specific tests to BRE Digest 365, carried out at the depth and location of the proposed soakaway or infiltration feature. Mapping and screening indicate where testing is worth doing; they do not replace it. AFC's planning-stage strategy assumed partial infiltration with no test (Issue 01, 18 June 2026, through a reduced runoff factor for the paving; Issue 02, 5 July 2026, at 1×10⁻⁵ m/s). The FRMO's response of 3 July 2026 found the strategy submitted at that time unacceptable, and gave the absence of site-specific infiltration testing as one of the reasons.

At the condition stage, infiltration was ruled out on the ground evidence. Infiltration potential, screened using the proprietary Air & Flood Consultants infiltration potential model, was considered to be moderate, with an indicative permeability of 1×10⁻⁷ to 1×10⁻⁵ m/s. BGS records a non-coal mining hazard in the area, and BGS mapping indicates River Terrace Deposits about 1 m thick over mudstone and sandstone of the Freshwater West Formation. DMRB CD 530 advises that soakaways are not to discharge into mine workings. The permeable paving was therefore fully lined and the model takes no credit for infiltration, so the design does not depend on a rate that nobody has measured. Whether the flood officer accepts a lined design without infiltration tests had not been decided when this guide was written: the council's list of what this condition needs included infiltration testing.

Surface water body. The nearest watercourse, the River Lyd, is about 330 m from the site with no available connection.

Surface water sewer. The Severn Trent records show none in Victoria Road or adjoining the site. Three footway manholes shown on the architect's drawing do not appear on the records and are considered to be highway drainage.

Combined sewer. The surface water is to discharge at a controlled rate to MH 2001 on the combined sewer, by gravity through a backdrop (Figure 6). A new connection to a public sewer needs the water company's approval under section 106 of the Water Industry Act 1991, and works in the highway need a street works permit. The surface water was not routed to the existing drain to MH 2007, because that drain is recorded as foul and the outlet of the lined paving sits about 1.1 m below the level at which the drain would be joined, so only a pumped connection could reach it.

Figure 6: The discharge point, proven from the sewer records with cover and invert levels (inset from drawing 112-DR-02 P6).

The FRMO accepted a condition at planning stage because a combined sewer was close by. A condition submission still has to show the connection: the manhole, its levels, the route and the consent required.

Step 3: set the discharge rate

The flood officer's starting point is usually the greenfield runoff rate. QBAR, the mean annual peak flow from the undeveloped site, is commonly estimated by the IH124 method. For a small site the number can be impracticably small. For the 0.037 ha Lydney site, the IH124 method gave QBAR 0.08 l/s, 1 in 1-year 0.07 l/s, 1 in 30-year 0.14 l/s and 1 in 100-year 0.17 l/s. The strategy states: "A flow control set to 0.08 l/s would require an orifice of less than 10 mm."

An orifice of that size blocks. The national standards address the point in two places. Standard 3.18 limits the peak rate in the 50% AEP event to "the equivalent 50% AEP greenfield runoff rate, or 3 l/s/ha, whichever is the greater", and standard 3.22 requires that where orifices are under 50 mm, "the flow control shall be robustly protected from blockage risks." At 0.037 ha, 3 l/s/ha is about 0.11 l/s, so the floor in the standards does not solve the problem on a site this small.

Local guidance often does. The Gloucestershire SuDS Design and Maintenance Guide (page 32) sets 5 l/s as the minimum practicable discharge rate for any outfall in the county, and notes that a 5 l/s cap is inappropriate where flows should not exceed 2 to 3 l/s. For previously developed and drained brownfield sites, section 5.3.2 of the guide states that "the SuDS systems should modify the runoff rate to achieve minimum of 40% reduction in peak discharges/volumes, but endeavor to reduce flows as close as reasonably practical to the greenfield rates for the same events." Where it is not reasonably practicable to control to the determined runoff rate, the application must show that "The volume of storage required would lead to the development becoming unviable; The volume of storage required could not be accommodated on the development site."

The Lydney strategy cites page 32 of the guide for two points: the 5 l/s minimum and the 2 to 3 l/s note. Its own design criteria require peak discharge to be reduced by at least 40% against the existing site in every event, and brought as close as reasonably practicable to the greenfield rate.

The rate chosen was a vortex flow control rated at 2.3 l/s at 0.47 m head. A vortex control passes its design flow through a larger opening than an orifice plate of the same rating; the equivalent orifice here is about 40 mm, and every inflow passes a catchpit and the paving sub-base before it reaches the control. The strategy justifies the rate on two grounds: it reduces the peak from the existing positively drained roofs and yard by between 72% and 90% across the five design events, above the strategy's own 40% minimum (Figure 7); and the orifice size needed for QBAR is impracticable.

Figure 7: Peak discharge, existing against proposed, for each design event. Redrawn from Table 8 of the Lydney strategy (EPA SWMM 5.2).

The strategy also reports the comparison against the existing roofs alone, 1.42, 5.26 and 6.73 l/s in the 1, 30 and 100-year events, which gives reductions of 37%, 70% and 73%. The FRMO's February 2026 response asked for discharge "in line with the QBAR greenfield runoff rate". Whether the FRMO accepts 2.3 l/s against that wording had not been decided when this guide was written. The general point stands: a rate above greenfield is presented with its justification, its source and the reduction against the existing site, and never left for the officer to query.

Step 4: size the storage

Storage is sized to the design criteria and to the climate change allowance, both stated. The Lydney criteria were no flooding from the drainage system in the 1 in 30-year event plus 35%, no flooding of buildings in the 1 in 100-year event plus 40% with flows above that contained on site, half of the storage to drain within 24 hours, and a 10% allowance for urban creep on all drained areas. The 35% and 40% figures are the Environment Agency upper end peak rainfall allowances for the 2070s epoch in the Severn Vale management catchment, taken from the climate change allowances guidance. The national standards (3.35 and 3.36) set the same pair of tests: no flooding on any part of the development up to the 3.3% AEP event, and no flooding of a building up to the 1% AEP event.

The storage is 134.1 m² of concrete block permeable paving over a lined sub-base with 30% effective porosity and a 450 mm storage depth, giving 18.1 m³ (Figure 8). The formation is laid level at 22.72 mAOD (assumed), so the full depth is available across the whole paved area, as modelled.

Figure 8: The storage, drawn as built: lined permeable paving section (Section A, drawing 112-DR-02 P6).

The calculations were run in EPA SWMM 5.2 as a pipe-by-pipe network model: 14 storm durations from 15 minutes to 24 hours, summer and winter profiles, 140 runs for each model. In the 1 in 100-year plus 40% event the sub-base reaches 419 mm, using 16.9 m³ of the 18.1 m³ available, the peak discharge is 2.22 l/s, and the sub-base half-empties within 1.5 hours of the peak (Figure 9). No water leaves the drainage system at the surface in any modelled event.

Figure 9: The model result for the design event, 1 in 100-year plus 40% with 10% urban creep, 120-minute summer storm.

Three results are reported alongside, because an officer checking the design looks for them. The lined system does not reduce runoff volume: about 23.1 m³ from the proposed roofs and paving in the 1 in 100-year, 6-hour event (25.4 m³ with urban creep), against 18.5 m³ from the existing roofs and yard, all released at no more than 2.22 l/s. The lined system provides no interception of the first 5 mm of rainfall from the roofs, which Standard 2 of the national standards requires: "Interception shall be delivered within the development for the first 5mm of rainfall for the majority of rainfall events" (clause 2.5). The water level in the flow control chamber rises to within 300 mm of its cover in the design event, so the calculation output flags it.

Step 5: the drawings

The proposed drainage layout is the document the contractor builds from and the officer approves. It shows every pipe with its size, gradient and invert levels, every chamber with cover and invert levels, the flow control with its rating, the connection to the outfall, and the exceedance route. On drawing 112-DR-02 each pipe carries the same reference as the calculations (links 1.000 to 4.001), so drawing and model can be read together (Figures 10 and 11).

Figure 10: Proposed drainage layout, surface water and foul (extract from drawing 112-DR-02 P6).

Figure 11: Flow control, outfall and exceedance route on the proposed layout (detail of drawing 112-DR-02 P6).

The exceedance route is part of the design. Beyond the design event, or if the flow control blocks, water would rise to the paving surface and follow the fall of the car park to its east corner. A 100 mm kerb upstand along the low boundaries holds it on site, and external levels are kept at least 150 mm below finished floor level at thresholds.

Step 6: water quality and the maintenance plan

Water quality. The CIRIA C753 simple index approach compares the pollution hazard of each surface with the mitigation the SuDS component provides. A low hazard car park scores 0.5, 0.4 and 0.4 for suspended solids, metals and hydrocarbons, a medium hazard car park 0.7, 0.6 and 0.7, and permeable paving provides 0.7, 0.6 and 0.7, sufficient in both cases. Roof water passes catchpits before it enters the sub-base.

Maintenance. The PPG (paragraph 060, Reference ID 7-060-20220825) states that "Proposals for Sustainable Drainage Systems should include arrangements for their long-term maintenance", and Standard 7.2 of the national standards requires a management and maintenance plan from the designer. The officer looks for two things: a named party responsible for the lifetime of the development, and a schedule of tasks and frequencies. At Lydney the drainage is private and is to be maintained by the owner of the premises for the lifetime of the development, with the obligation passed to any tenant through the lease. The strategy's Management and Maintenance section, summarising its operation and maintenance plan, sets out sweeping of the paving once a year after leaf fall, inspection of the catchpits and the flow control chamber every six months and after any storm causing standing water, and a set of restrictions: no sealing or resurfacing of the paving, no storage of materials on it, and no excavation without reinstating the liner. Figure 12 reproduces the plan's schedule for the paving.

Figure 12: A maintenance schedule the owner can follow (Table 15 of the Lydney strategy's operation and maintenance plan).

Foul water: the part of the condition that gets left out

Many drainage conditions cover foul water too, and a surface water strategy alone does not discharge them. The foul section states where the foul water goes, whether a new public sewer connection or any other consent is needed, and the pipe sizes and gradients. At Lydney the new foul drains use the building's existing outfall to MH 2007, with no new public connection, and the peak foul flow is 2.8 l/s by the discharge unit method of BS EN 12056-2. The roof water connections to that drain are to be removed, so that it carries foul water only. Dental chair waste passes an amalgam separator of at least 95% retention, as Regulation (EU) 2017/852, now assimilated law, requires for dental facilities.

The traps

These are the points on which drainage condition submissions are most often returned:

• Infiltration assumed, not tested. An infiltration rate without BRE Digest 365 tests is a design that cannot be checked. Either test before submission, or line the system and take no infiltration credit with the reasons recorded, as at Lydney. A lined design without tests may still be questioned: the council's list for the Lydney condition included infiltration testing, and the flood officer had not ruled on the lined design when this guide was written.

• The outfall left open. "Discharge to the public sewer, subject to agreement" is not a discharge point. Show the manhole, its levels and the consent needed.

• A rate with no justification. A rate above greenfield needs its reasoning: the orifice size QBAR would need, the local guide's minimum, and the reduction against the existing site in every event.

• No layout, or a layout without levels. A layout drawing without invert levels, pipe sizes and the flow control cannot be built from or approved.

• No maintenance owner. "To be maintained" without a named responsible party and a schedule does not meet paragraph 060 of the PPG.

• The planning-stage strategy resubmitted. AFC's planning-stage strategy for Lydney (Issues 01 and 02) assumed about 180 m² of paving with a 300 mm sub-base and partial infiltration. The condition-stage design re-measured the site (134.1 m² of paving), lined the system and deepened the storage to 450 mm. A condition is discharged on the design to be built.

• Levels not surveyed. Levels taken from LiDAR are a valid starting point when stated as assumed. A topographic survey before construction, and a re-check of the hydraulics if it changes the outfall level, should be listed in the submission.

• Roof water left in a foul drain. Where the existing roofs drain to a foul sewer, the strategy states it and removes the connection.

What to submit

A drainage condition submission normally contains:

• a covering table that sets each requirement of the condition, and each flood officer comment from the planning file, against the section or drawing that answers it;

• an existing drainage layout, from a CCTV survey or a desk-based record with its assumptions stated, with the water company's sewer records;

• the drainage hierarchy worked through level by level, with BRE Digest 365 infiltration test results where infiltration is proposed;

• the discharge rate and its justification, greenfield rates by IH124 or a similar method, and the local guide's requirements;

• hydraulic calculations for the design events with the climate change allowance and urban creep stated, and the results for storage depth, volume used, peak discharge and half-drain time;

• a proposed drainage layout with pipe sizes, gradients, invert and cover levels, the flow control, the outfall and the exceedance route, plus sections through the SuDS components;

• the foul drainage design, where the condition covers foul water;

• a water quality assessment (CIRIA C753 simple index approach for most small sites);

• a management and maintenance plan naming the responsible party for the lifetime of the development;

• a list of the information to be confirmed before construction, such as surveys and the sewer connection consent.

The difference between a drainage strategy and a flood risk assessment, and which one a council is asking for, is set out on our page on drainage strategies and flood risk assessments. The background to SuDS components is in our post on SuDS and why they are not going away, and a decided example of a combined flood risk assessment and SuDS strategy is the Harrow Weald case study.

Where the Lydney condition stands

Planning permission P1568/25/FUL was granted with conditions by Forest of Dean District Council on 18 August 2026 (application record). AFC prepared the planning-stage SuDS strategy (Issue 01, 18 June 2026; Issue 02, 5 July 2026) and the revised condition-stage SuDS strategy and drainage drawings in October 2026; the version described here is dated 27 October 2026. The discharge of condition 10 had not been decided when this guide was written, on 30 October 2026, and the guide will be updated with the outcome.

Frequently asked questions

What do I need to discharge a drainage condition?

A drainage strategy that answers each requirement of the condition and each flood officer comment on the planning file: the existing drainage, the discharge point proven by the drainage hierarchy, the discharge rate and storage with calculations, a proposed drainage layout with levels, the exceedance route and a maintenance plan. The full list is in the What to submit section above.

How long does it take to discharge a pre-commencement drainage condition?

The council has 8 weeks from a valid application to decide, or longer if agreed in writing, under the Planning Practice Guidance on planning conditions. Preparing the strategy takes longer if surveys or infiltration tests are needed first, and a submission returned with questions adds a further round.

Can I start work before the drainage condition is discharged?

Not if it is a pre-commencement condition. The Planning Practice Guidance states that development carried out without having complied with a pre-commencement condition would be unlawful and may be the subject of enforcement action. Read the wording of the condition, and what it says must happen before which stage of the works, before anything starts on site.

Do I need infiltration tests to discharge a SuDS condition?

If the design relies on infiltration, yes: a soakaway or infiltration feature needs site-specific tests to BRE Digest 365. Where tests cannot be done, or the ground is unsuitable, the system can be lined and designed to discharge elsewhere with no infiltration credit, with the reason recorded against the drainage hierarchy. The council may still ask for tests. At Lydney the council's list for the condition included infiltration testing, and the flood officer had not ruled on the lined design when this guide was written. Our infiltration potential map indicates where testing is worth doing.

What if the greenfield runoff rate is too small for a flow control?

State the problem with its numbers and propose a practicable rate with its justification. At Lydney, QBAR was 0.08 l/s, which needs an orifice under 10 mm. The strategy used a 2.3 l/s vortex control (equivalent orifice about 40 mm) behind catchpits, cited the county guide's 5 l/s minimum and set a 40% minimum reduction against the existing site as its own design criterion, and showed a 72% to 90% reduction against the existing site in every event.

Can I connect surface water to a combined sewer?

Yes, as the last level of the drainage hierarchy, once infiltration, a watercourse and a surface water sewer have been ruled out with evidence. The connection needs the water company's approval under section 106 of the Water Industry Act 1991, and the discharge is normally restricted by a flow control.

Who has to maintain the SuDS after the condition is discharged?

Whoever the maintenance plan names, for the lifetime of the development. On most small sites that is the owner, with the duty passed to tenants and successors. Many conditions also require the system to be maintained "thereafter", so a lapse in maintenance can be a breach of condition.

Ask us about your drainage condition

AFC prepares SuDS strategies, drainage calculations and drainage layouts for condition discharge, for householder, commercial and residential schemes. Send us your decision notice through the fee proposal form and we will tell you what the condition needs. More on the service is on our SuDS strategy page.