Flood Risk Assessment — Flood Zone 2 Extension with Reservoir and Groundwater Risk, Datchet

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 | Client: Jagdeep Singh | Services: Flood Risk Assessment

Three Sources, One Design Response

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.

Assessing three elevated sources separately and then designing three separate mitigation packages would produce a disproportionate and largely unbuildable scheme. The value of the assessment lies in recognising that all three sources produce the same failure mode — water at or near ground level for a period — and that a single coherent design response can address them together.

The Site and the Proposals

We were appointed by Jagdeep Singh 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, with the Environment Agency consulted in August 2026.

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

Two measures carry the mitigation. First, all bedroom accommodation is located at first floor, with none at ground floor level, and finished floor levels to those rooms are set at least 300mm above the 1-in-100-year plus 35% climate change flood level. Placing sleeping accommodation above the design flood level is the single most effective response available on a site of this character, because it addresses fluvial inundation, groundwater emergence and reservoir breach simultaneously — and it removes the scenario in which occupants are asleep at the level where water arrives.

Second, all below-ground structures — foundations and retaining walls — are constructed in waterproof concrete with sealants at joints and penetrations, addressing the high groundwater rating directly and providing resistance during any period of standing water at ground level.

Internally, moisture-resistant materials and raised electrical services are specified at ground floor, 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.

Outcome

The FRA demonstrated that the extensions are at an acceptable level of flood risk for their lifetime and that flood risk to the site and to surrounding properties is not increased. It also produced a proportionate specification: one floor-level decision and one construction decision, addressing three separate elevated risk sources without loading the scheme with duplicated mitigation.