Flood Risk Assessment, SuDS Strategy and Sequential Test — Subdivision into Two Flats, Egham
We prepared a Flood Risk Assessment, SuDS Strategy and Flood Risk Sequential Test for the change of use and rebuild of 3 Grange Road, Egham — a Flood Zone 2 site in Runnymede — on behalf of Fiona Jones of Cameron Jones Planning Ltd. The scheme rebuilds on the same footprint and subdivides a dwellinghouse with ground floor offices into two self-contained flats: Flat 1 at 56.6m² GIA and Flat 2, a maisonette of 17.6m² at ground floor and 81.0m² at first floor. Fluvial risk is medium and sewer risk medium, while groundwater and artificial-source risks are both high, reflecting the Thames valley gravels and the reservoir breach inundation extent. The rebuild cuts impermeable area from approximately 276.6m² to 158.1m², a reduction of around 43%, and the drainage network is designed to the 1-in-100-year event with a 40% climate change allowance. The application is registered under reference RU.26/0845.
Flood Risk Assessment, SuDS Strategy and Sequential Test for Subdivision into Two Flats, 3 Grange Road, Egham
Location: 3 Grange Road, Egham, Surrey, TW20 9QW | Client: Fiona Jones, Cameron Jones Planning Ltd | Services: Flood Risk Assessment; SuDS Strategy; Flood Risk Sequential Test
Why a Same-Footprint Rebuild Needed a Sequential Test
A rebuild on the same footprint sounds like it should sit outside the Sequential Test. In practice, the change of use is what engages it. Converting a dwelling with ground floor offices into two self-contained flats increases the number of people sleeping at the site and changes the vulnerability classification of part of the floorspace. In Flood Zone 2, that combination brings the proposal within scope.
We therefore prepared the Sequential Test alongside the FRA rather than leaving it to be argued informally. Setting the area of search from the development's anticipated catchment, screening reasonably available alternative sites and documenting why each is unsuitable is work best done once, properly, at submission — rather than in response to a holding objection three months later.
The Site and the Proposals
We were appointed by Fiona Jones of Cameron Jones Planning Ltd to prepare a Flood Risk Assessment, a SuDS Strategy and a Flood Risk Sequential Test for 3 Grange Road, Egham, Surrey, TW20 9QW, in support of application RU.26/0845.
The proposal is the change of use and rebuild of the existing building on the same footprint, with no increase in footprint, sub-dividing the existing single dwellinghouse — currently a dwelling with offices at ground floor level — into two self-contained flats. Flat 1 occupies the ground floor at 56.6m² GIA. Flat 2 is a maisonette of 17.6m² at ground floor and 81.0m² at first floor.
Flood Risk Profile
Egham sits on the Thames floodplain in Runnymede, close to the confluence of the Thames with the Bourne and within the influence of the Thames Valley reservoir chain. That setting produces the site's distinctive risk profile, in which the flood zone is not the most demanding rating the site carries.
Fluvial risk is medium, consistent with Flood Zone 2 — within the 1-in-1000-year extent, outside the 1-in-100-year extent. Surface water risk is low, with no significant modelled ponding across the plot. Sewer flooding risk is medium, reflecting recorded incidents in the surrounding postcode sector. Tidal risk is low.
Groundwater risk is high, reflecting the permeable Thames valley gravels beneath the site and their hydraulic connection to the river. Artificial-source risk is high, reflecting the reservoir breach inundation extent that covers much of this part of Runnymede.
Drainage Strategy
The rebuild delivers a substantial reduction in impermeable area, from approximately 276.6m² to 158.1m² — a reduction of around 43%. That alone produces significant betterment in peak surface water discharge, and the SuDS strategy builds on it with source control and permeable surfacing appropriate to a site where the high groundwater rating limits reliance on infiltration. The network is designed to the 1-in-100-year event with a 40% climate change allowance.
Setting the baseline honestly is what makes the reduction usable. Where the existing arrangement discharges from a largely hard-surfaced plot without attenuation, quantifying both the before and after areas turns a general claim of improvement into a figure the Lead Local Flood Authority can check.
Flood Mitigation and Resilience
Finished floor levels are set to provide freeboard above the design flood level. Below-ground elements are specified in waterproof concrete with sealants at joints and penetrations, addressing the high groundwater rating. Ground floor construction uses moisture-resistant finishes and raised electrical services, and the first floor accommodation within Flat 2 provides safe refuge above the design flood level. Occupants are directed to register with the Environment Agency Floodline warning service.
Outcome
The three documents together give Runnymede Borough Council a complete flood risk position: the Sequential Test addressing the policy question of whether the development should be here, the FRA addressing safety and betterment, and the SuDS Strategy quantifying a 43% reduction in impermeable area on a constrained, groundwater-sensitive Thames valley plot.