Flood Risk Assessment and SuDS Strategy — Office to Residential Conversion, East Sheen
We provided a combined Flood Risk Assessment and SuDS Strategy for an office-to-residential conversion in East Sheen, London Borough of Richmond upon Thames. The site sits on London Clay with groundwater flood susceptibility of 75% or higher, immediately ruling out infiltration drainage under the SuDS hierarchy.
Our design centred on a multi-component management train: a 41.5m² sedum green roof, 630 litres of rainwater harvesting storage, 118.8m² of permeable paving, and an 8.9m³ geocellular attenuation tank discharging at a controlled rate to the surface water sewer. Modelled to the 1-in-100-year event with 40% climate change allowance, the scheme achieved an 83% reduction in peak discharge — from 16.63 l/s to 2.84 l/s. Planning permission was granted in full.
Flood Risk Assessment and SuDS Strategy — Office to Residential Conversion, East Sheen
Client: SheenPhysio | Location: East Sheen, London Borough of Richmond upon Thames | Services: Flood Risk Assessment, SuDS Strategy
Project Background
Converting a commercial office building to three residential dwellings sounds straightforward — until the ground conditions tell a different story. The East Sheen site sits on London Clay Formation with groundwater flood susceptibility classified at 75% or higher under Richmond's Strategic Flood Risk Assessment, making robust surface water management essential from the outset.
The proposed development involved a two-storey side extension, roof extension with dormers, and a single-storey rear extension. Although the scheme actually reduced impermeable area from 314m² to 212m², the change of use from commercial to residential triggered a requirement to demonstrate comprehensive flood risk management to the satisfaction of the Local Planning Authority.
Flood Risk Assessment
We carried out a site-specific flood risk assessment covering all relevant flood sources — fluvial, pluvial, groundwater, sewer, and artificial. The Environment Agency Flood Map confirmed Flood Zone 1 classification, ruling out significant fluvial risk. The primary concern, however, was groundwater.
Borehole records from a nearby investigation (approximately 140m east of the site) indicated gravel and sand with clay extending to 6–8 metres depth. Soilscapes mapping corroborated this, classifying the soils as loamy with naturally elevated groundwater. This ground condition immediately ruled out infiltration-based drainage — the preferred approach under the SuDS hierarchy — and shaped the entire drainage design strategy.
SuDS Strategy
With infiltration demonstrably unfeasible, we designed a multi-component SuDS management train that addresses source control, conveyance, and attenuation in sequence.
At source, a 41.5m² sedum green roof provides rainfall interception and biodiversity benefit, whilst three 210-litre water butts (630 litres combined storage) capture roof runoff for garden irrigation. Strategic tree planting across each garden plot maximises canopy interception and contributes to long-term amenity.
For conveyance and attenuation, 118.8m² of permeable paving with a 300mm sub-base drainage layer manages surface runoff across the hardstanding areas. An 8.9m³ geocellular attenuation tank (25% porosity) provides primary storage, with controlled discharge to the existing surface water sewer at a rate that significantly undercuts existing conditions.
The system was modelled for the 1-in-100-year storm event with a 40% climate change allowance applied. Peak discharge was reduced from 16.63 l/s under existing conditions to 2.84 l/s post-development — an 83% reduction, and well below the calculated Qbar greenfield rate of 0.1 l/s.
Groundwater Resilience Measures
Given the site's high groundwater susceptibility, the flood resilience specification addressed below-ground structural risk directly. Waterproof concrete was specified for all below-ground elements, with electrical sockets positioned at a minimum of 1.0m above finished floor level. Sealed thresholds, appropriately raised air bricks, and moisture-resistant building materials throughout the structure provide layered protection. Sump pump provisions were also incorporated for construction phase groundwater monitoring.
Outcome
Planning permission was granted by the London Borough of Richmond upon Thames. The assessment demonstrated compliance with National Planning Policy Framework requirements, Richmond Local Plan Policy LP 21, and London Plan Policy SI 13. All flood risk to future occupants was shown to be acceptable, with no increase in flood risk to neighbouring properties, and clearly defined long-term maintenance responsibilities for the SuDS infrastructure.
Technical Summary
ParameterValueSite areaEast Sheen, London Borough of RichmondFlood ZoneZone 1 (EA Flood Map)Groundwater susceptibility75%+ (Richmond SFRA)Pre-development impermeable area314m²Post-development impermeable area212m²Pre-development peak discharge16.63 l/sPost-development peak discharge2.84 l/sDischarge reduction83%Design storm1-in-100-year + 40% climate changeGeocellular attenuation volume8.9m³