Flood Risk Assessment — Flood Zone 2 Extension with Reservoir and Groundwater Risk, Datchet
We prepared a site-specific Flood Risk Assessment for a front porch, part single and part two storey side and rear extension, hip-to-gable roof alteration and rear dormer at 26 Lawn Close, Datchet, for a private client. The Flood Zone 2 site carries three elevated ratings: medium fluvial risk, high groundwater risk from the permeable Thames valley gravels and their connection to the river, and high artificial-source risk from the reservoir breach inundation extent; surface water and sewer risks are low. The July 2026 FRA placed all bedroom accommodation at first floor with finished floor levels to those rooms at least 300mm above the 1-in-100-year plus 35% climate change flood level, and specified waterproof concrete with sealants to all below-ground structures. The application was validated on 30 July 2026 under reference 26/01517/FULL and refused on 15 September 2026, in part on flood risk. AFC then prepared a new FRA for a revised scheme using the EA's modelled flood levels.
Flood Risk Assessment for Extensions on a Flood Zone 2 Site with Groundwater and Reservoir Breach Risk, 26 Lawn Close, Datchet
Location: 26 Lawn Close, Datchet, Slough, SL3 9JZ | Services: Flood Risk Assessment
Three Elevated Flood Sources on a Flood Zone 2 Site
Datchet sits on the Thames floodplain between Windsor and Staines, downstream of a chain of impounding reservoirs and above a highly permeable gravel aquifer. That setting produces an unusual risk profile for a village address: a Flood Zone 2 fluvial designation that is not, on its own, the most demanding constraint, alongside high groundwater risk and high risk from artificial sources.
The July 2026 FRA rated three sources as elevated at the site: fluvial flooding medium, groundwater flooding high and flooding from artificial sources high. Surface water and sewer flooding were rated low. The FRA set out a separate package of mitigation measures for each of the three elevated sources.
The Site and the Proposals
We were appointed by a private client to prepare a site-specific Flood Risk Assessment for a front porch, part single and part two storey side and rear extension, hip-to-gable roof alteration and rear dormer at 26 Lawn Close, Datchet, Slough, SL3 9JZ, in the Royal Borough of Windsor and Maidenhead. The application was validated on 30 July 2026 under reference 26/01517/FULL. The Environment Agency, consulted in August 2026, returned the consultation as outside its remit, leaving the council to apply the standing advice itself.
The site occupies a residential plot in an established close, bounded by dwellings and gardens, with broadly uniform ground levels and no ordinary watercourse in the immediate vicinity. The proposals constitute minor householder development, so the Sequential and Exception Tests are not engaged and the assessment addresses safety over the lifetime of the development and the effect on flood risk elsewhere.
Evidence Base
We drew on the Environment Agency Flood Map for Planning, the Risk of Flooding from Rivers and the Sea and Surface Water Spatial Planning datasets, the EA reservoir flood inundation mapping, BGS 1:50,000 geological mapping, EA LiDAR composite terrain data, the recorded sewer flooding history for the postcode and the Royal Borough of Windsor and Maidenhead Strategic Flood Risk Assessment.
Flood Risk Profile
Fluvial risk is medium: the site falls within the 1-in-1000-year extent but outside the 1-in-100-year extent on the EA Flood Map for Planning. Surface water risk is low, with no significant modelled ponding across the plot. Sewer flooding risk is low, with no incident recorded at the development address. Tidal risk is low.
Groundwater risk is high, reflecting the permeable Thames valley gravels beneath the site, their hydraulic connection to the river, and recorded groundwater levels in the wider area. In a Thames flood event, groundwater emergence and out-of-bank flow occur together rather than as independent scenarios.
Artificial-source risk is high. The site falls within the modelled inundation extent associated with reservoir breach. That is a residual rather than a probable risk, but it is one the assessment has to address on its own terms rather than dismiss on grounds of low likelihood.
Flood Mitigation and Resilience
The July 2026 FRA placed all bedroom accommodation at first floor, with none at ground floor level, and set finished floor levels to those rooms at least 300mm above the 1-in-100-year plus 35% climate change flood level. All below-ground structures, such as foundations and retaining walls, were to be constructed in waterproof concrete with sealants, addressing the high groundwater rating.
At ground floor, the FRA specified solid concrete floors with resilient finishes, resilient skirtings, door frames and window sills, and sealed service entry points, and the occupants are directed to register with the Environment Agency Floodline warning service so that the warning lead time available on this reach of the Thames can actually be used.
The case officer did not accept the floor level approach: the Environment Agency's householder standing advice measures freeboard from the ground floor, and the report gave no design flood level and no floor levels in mAOD.
Outcome
Application 26/01517/FULL was refused on 15 September 2026, on the design of the rear roofscape and on flood risk: the council found the flood risk modelling insufficient and the scheme not compliant with the Environment Agency's householder standing advice. AFC then prepared a new FRA for a revised scheme using the EA's modelled flood levels: a design flood level of 18.93 mAOD, a ground floor level of 19.45 mAOD and flood resistance and resilience to 19.53 mAOD. The revised scheme had not been submitted at the time of writing.