Flood Risk Assessment — Flood Zone 2 Rear Extension with High Groundwater Risk, Maidenhead

Flood Risk Assessment for a Single Storey Rear Extension in Flood Zone 2, 1 Partridge Mead, Maidenhead
Location: 1 Partridge Mead, Maidenhead, Berkshire, SL6 7HG | Client: Phil Hurdwell, PJH Design | Services: Flood Risk Assessment

Groundwater, Not the River, Sets the Specification

Maidenhead sits within the Thames corridor, and on a Flood Zone 2 householder application it is normally the fluvial designation that draws attention. At 1 Partridge Mead the more demanding constraint was groundwater. The Thames valley gravels that underlie much of the town are highly permeable and hydraulically connected to the river, so the water table responds directly to river levels and can emerge at or near ground level in prolonged wet periods, well outside the modelled fluvial extent.

For a single storey rear extension that distinction is not academic. It determines what the finished floor level has to be, how the below-ground elements are built, and what happens to any water that reaches the structure. An assessment that treated the Flood Zone 2 designation as the governing issue would have specified freeboard and stopped there, leaving the source most likely to affect the building unaddressed.

The Site and the Proposals

We were appointed by Phil Hurdwell of PJH Design to prepare a site-specific Flood Risk Assessment for a single storey rear extension at 1 Partridge Mead, Maidenhead, Berkshire, SL6 7HG, in the Royal Borough of Windsor and Maidenhead, in support of the planning application submitted under reference 26/01427/FULL.

The site occupies a residential plot within an established estate, bounded on all sides by dwellings and gardens. Ground levels across the plot are broadly uniform and there are no ordinary watercourses in the immediate vicinity. The proposals constitute minor householder development, so the Sequential and Exception Tests are not engaged, and the assessment turns on whether the development is safe for its lifetime and whether it increases flood risk elsewhere.

Evidence Base

We drew on the Environment Agency Flood Map for Planning, the Risk of Flooding from Rivers and the Sea dataset and the Surface Water Spatial Planning products, together with 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. Each of the six flood sources was rated against that evidence rather than against the flood zone alone.

Flood Risk Profile

Fluvial risk is medium, consistent with the Flood Zone 2 designation: the site falls within the extent of the 1-in-1000-year event but outside the 1-in-100-year extent. Surface water risk is low, with no significant modelled ponding across the plot and no overland flow route crossing it. Sewer flooding risk is low, with no incident recorded at the development address. Artificial-source risk is low, and tidal risk is low, the site lying far upstream of the tidal limit.

Groundwater risk is high. The rating reflects the permeable Thames valley deposits beneath the site and recorded groundwater levels in the wider area, and it is the source that sets the construction specification.

Mitigation and Resilient Construction

Finished floor levels within the extension are set to provide freeboard above the modelled design flood level, giving margin against both the fluvial event and groundwater emergence.

All below-ground structures — foundations, retaining elements and slab edges — are specified in waterproof concrete with sealants at construction joints and service penetrations. That is the appropriate response to a high groundwater rating on a granular deposit, where the loading is hydrostatic and sustained rather than a short peak. Internally, moisture-resistant materials are specified at ground floor level: resilient plaster or moisture-resistant plasterboard, low water-absorption insulation, and well-sealed pipework with minimal service voids.

Infiltration drainage was not relied upon as the primary surface water disposal route. A high groundwater rating removes the assumed infiltration capacity in precisely the conditions under which the system would be required to perform.

Outcome

The FRA demonstrated that the extension is at an acceptable level of flood risk for its lifetime and does not increase flood risk to the site or to surrounding properties. The Royal Borough of Windsor and Maidenhead granted planning permission on 6 August 2026 under reference 26/01427/FULL. The commission illustrates a point that recurs across the Thames valley: on a Flood Zone 2 site, the groundwater rating often does more to shape the build than the fluvial one.