62 St Johns Road, Sevenoaks

Project Background
A homeowner sought planning permission for substantial alterations and extensions to a semi-detached dwelling in a traditional residential neighbourhood in Sevenoaks, Kent. The proposals comprised a new rear extension incorporating basement-level accommodation, ground floor expansion, and renovations to existing first and second floor spaces.
Air & Flood Consultants was commissioned to prepare a comprehensive Flood Risk Assessment to support the planning application. The assessment was required to demonstrate compliance with the National Planning Policy Framework (NPPF) and satisfy Sevenoaks District Council's local planning requirements, particularly given the site's identification within groundwater flood risk areas on the Council's Strategic Flood Risk Assessment mapping.
Site Context and Constraints
The site is located at grid reference E: 553016; N: 156334 within a predominantly residential area characterised by traditional semi-detached housing and small-scale private gardens. The property occupies previously developed brownfield land, currently containing an existing residential building, and is bounded by dwellings on all sides, being well integrated into the established urban fabric.
Topographical analysis using LiDAR data indicated relatively uniform ground levels across the site, consistent with the surrounding urban landscape. The flat topography suggested good surface water drainage to existing infrastructure but provided no natural gradient for gravity drainage of any proposed basement.
Geological investigation revealed site conditions of particular relevance to basement construction and flood risk. British Geological Survey mapping confirmed the site is underlain by Folkestone Formation sandstone bedrock with no recorded superficial deposits. Borehole data from approximately 900 metres north-east of the site indicated sand with occasional fine gravel. Soilscapes mapping characterised the site as having freely draining slightly acid loamy soils, suggesting good infiltration potential for surface water but also indicating hydraulic connectivity with underlying aquifers—a critical consideration for groundwater flood risk.
The site was not in proximity to any main rivers, ordinary watercourses, or open drainage channels, being located entirely within an established urban drainage network served by combined sewers.
Technical Assessment Methodology
The Flood Risk Assessment followed established best practice guidance, drawing upon multiple authoritative data sources:
Regulatory Framework:

National Planning Policy Framework (NPPF)
Planning Practice Guidance (PPG)
Sevenoaks District Council Core Strategy (2011)
Strategic Flood Risk Assessment (2024)

Datasets and Mapping:

Environment Agency Flood Map for Planning
Surface Water Flood Risk Mapping (1 in 30, 1 in 100, and 1 in 1000-year events)
Sevenoaks SFRA Groundwater Pathways Mapping
Reservoir Inundation Mapping
LiDAR topographical survey data
British Geological Survey drift and geology maps
Historic flooding records

The assessment systematically evaluated all potential flood sources in accordance with NPPF requirements: tidal flooding, fluvial flooding, surface water (pluvial) flooding, groundwater flooding, sewer flooding, and flooding from artificial sources such as reservoirs.
Key Findings
Fluvial Flood Risk (Rivers and Sea): The Environment Agency's Flood Map for Planning confirmed the entire site lies within Flood Zone 1, indicating land with less than 0.1% annual probability of flooding from rivers or the sea. The site lay well outside the design flood extent for the 1 in 100-year plus climate change event shown on Environment Agency climate change mapping. Fluvial flood risk was classified as low, and Sequential and Exception Tests were not required.
Surface Water (Pluvial) Flood Risk: Environment Agency surface water flood mapping for 1 in 30-year, 1 in 100-year, and 1 in 1000-year events indicated the site lies outside any identified areas of elevated surface water flood risk. Whilst some localised surface water risk was evident in the wider area including adjacent roads and footways, these risks did not directly affect the development site. The proposed extension would be situated on land remaining outside modelled extents of pluvial flood risk. Surface water flood risk was classified as low.
Groundwater Flooding: This represented the most significant flood risk to the proposed development. Review of Sevenoaks District Council's Strategic Flood Risk Assessment (2024) groundwater flood mapping revealed the site lies within an identified groundwater emergence area. These zones indicate locations where the water table may rise to or above ground level during prolonged wet periods, typically associated with permeable geology such as the Folkestone Formation sandstone underlying the site.
The combination of sandy bedrock, freely draining soils, and location within a mapped groundwater pathway zone indicated a high likelihood that groundwater levels could rise during extended periods of above-average rainfall. Such conditions pose particular risk to below-ground structures including basements, which can be subject to hydrostatic uplift pressures, water ingress through walls and floors, and structural damage if not appropriately designed and constructed.
No site-specific records or historical incidents of groundwater flooding had been identified within or immediately adjacent to the site. However, the absence of historic flooding does not preclude future risk, particularly given climate change projections for increased winter rainfall intensity and duration. Groundwater flood risk was classified as high.
Sewer Flooding: The SFRA identified 49 recorded sewer flood incidents across Sevenoaks District since 2011, but the site was not located in one of the specific areas of elevated sewer flood risk. Sewer flooding risk was classified as low.
Artificial Sources (Reservoirs): Reservoir breach flood risk mapping for both wet-day and dry-day scenarios confirmed the property lay outside any potential inundation zones. Risk from artificial sources was classified as low.
Historic Flooding: Review of available records identified historic flooding incidents elsewhere within Sevenoaks District, but no Environment Agency records of past flood events at this specific location. The presence of flooding elsewhere in the district reinforced the importance of comprehensive flood risk assessment for all new development.
Mitigation Strategy
Given the identified high risk of groundwater flooding to the proposed basement construction, the FRA recommended a comprehensive suite of mitigation and resilience measures:
Groundwater Flood Resilience for Basement Construction:

Structural Waterproofing: All below-ground structures including basement walls, floor slab, and any retaining walls must be constructed using waterproof concrete conforming to BS 8102:2009 standards. This should incorporate:

Type A (barrier) protection through the use of waterproof concrete mixes
Type B (structurally integral) protection via appropriate reinforcement detailing
Type C (drained) protection systems where appropriate
Comprehensive tanking membranes to walls and floor


Groundwater Monitoring During Construction: Groundwater levels should be monitored throughout excavation and construction phases. If groundwater ingress is observed during excavation, temporary dewatering systems (sump pumps or wellpoint dewatering) should be deployed to maintain dry working conditions and prevent damage to partially completed structures.
Permanent Drainage Provisions: The completed basement should incorporate:

Internal perimeter drainage channels connected to a sump chamber
Automatic sump pump system with battery backup to handle any residual seepage
High-level alarms to alert occupants to pump failure or exceptional water ingress
Annual maintenance regime for all pumping equipment


Flood-Resistant Finishes: Internal finishes within the basement should utilise moisture-resistant materials to facilitate rapid recovery in the event of any water ingress:

Moisture-resistant plasterboard or cement-based renders
Sealed concrete or tiled floor finishes rather than timber or carpet
Electrical installations positioned above potential flood levels where practicable



These measures ensure that the risk of groundwater flooding to the basement is reduced to an acceptable level, protecting both the structural integrity of the building and the safety and wellbeing of future occupants.
Policy Compliance and Planning Outcomes
The assessment demonstrated full compliance with the NPPF's flood risk requirements. As the site was located entirely within Flood Zone 1, the Sequential Test (steering development to areas of lowest flood risk) was not required. However, the NPPF explicitly requires flood risk assessments for development in Flood Zone 1 that may be subject to other sources of flooding where the development would introduce a more vulnerable use—a provision directly applicable to basement construction in an area of groundwater flood risk.
The FRA satisfied the NPPF's core principles by demonstrating that:

Future occupants would remain safe throughout the development's lifetime through implementation of appropriate waterproofing and drainage measures
The development would not increase flood risk either on-site or elsewhere (the basement construction would not impede groundwater flow or reduce flood storage)
Where practicable, opportunities to reduce overall flood risk were incorporated through enhanced surface water management

The report provided Sevenoaks District Council's planning officers with robust technical evidence to support their determination. By identifying the specific groundwater flood risk and proposing proportionate, achievable mitigation measures, the FRA enabled informed decision-making whilst protecting future occupants and neighbouring properties.
Professional Standards
The assessment was prepared in accordance with Chartered Institution of Water and Environmental Management (CIWEM) standards, demonstrating:

Systematic evaluation of all relevant flood sources with particular focus on site-specific risks
Use of current, authoritative datasets including up-to-date local authority SFRA mapping
Transparent methodology with fully referenced technical approaches
Proportionate and achievable mitigation recommendations appropriate to residential construction
Clear communication of residual risks and required ongoing maintenance

Lessons and Transferable Insights
This case study demonstrates several principles applicable to residential extensions incorporating basement accommodation in areas of groundwater flood risk:

Geology Matters: Sites underlain by permeable geology (sandstone, chalk, sand and gravel) require particular scrutiny for groundwater flood risk. Freely draining soils, whilst beneficial for surface water management, often indicate hydraulic connectivity with underlying aquifers.
Local Knowledge is Critical: Strategic Flood Risk Assessments prepared by local authorities contain essential site-specific information not available from national datasets. The Sevenoaks SFRA groundwater mapping provided crucial evidence informing risk classification and mitigation design.
Basements Require Enhanced Protection: Below-ground accommodation in areas of groundwater risk demands multiple layers of protection—structural waterproofing, active drainage systems, and flood-resilient finishes. No single measure is sufficient; a combined approach is essential.
Proportionate Assessment: For small-scale residential extensions, flood risk assessment must balance technical rigour with cost-effectiveness and accessibility. Detailed hydrological modelling was not required; reliance on established mapping datasets and application of best practice construction standards provided sufficient evidence for planning determination.
Maintenance is Essential: Flood resilience measures, particularly active drainage systems like sump pumps, require ongoing maintenance to remain effective. FRAs should clearly communicate this requirement to applicants and local authorities.