Flood Risk Assessment for a Flood Zone 2 Rear Extension, 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 | Services: Flood Risk Assessment

A Flood Zone 2 Extension with a High Groundwater Rating

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 FRA rated fluvial risk as medium and groundwater risk as high. The groundwater rating rests on the groundwater flood mapping in the Royal Borough of Windsor and Maidenhead Strategic Flood Risk Assessment (2017), which places the site in an area more than 75% susceptible to groundwater flooding. No site-specific records or historical incidents of groundwater flooding were identified within or adjacent to the site.

The two ratings led to two sets of measures in the FRA: flood resistance and resilience measures against fluvial flooding, and waterproof below-ground construction with groundwater monitoring during excavation.

The Site and the Proposals

We were appointed 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 bounded on all sides by existing dwellings. LiDAR data indicates that ground levels across the plot are relatively uniform, and there are no main rivers, ordinary watercourses or open drainage channels in close proximity. The proposals constitute minor householder development, so the Sequential and Exception Tests are not required, 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 and the Environment Agency's surface water and reservoir flood mapping, LiDAR terrain data, the site topographical survey, Ordnance Survey and DEFRA Magic mapping, soil mapping, British Geological Survey geological mapping and borehole records, the Borough Local Plan 2013 to 2033 and the Royal Borough of Windsor and Maidenhead Level 1 Strategic Flood Risk Assessment (2017). Each of the six flood sources was rated against that evidence.

BGS mapping indicates that the site is underlain by the Seaford Chalk Formation and Newhaven Chalk Formation (undifferentiated), with superficial deposits of the Taplow Gravel Member, predominantly sand and gravel. The nearest borehole record, about 140 m east of the site (BGS reference SU88SE112), suggests sandy clay and clay with sand and gravel.

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, and Environment Agency mapping shows it outside the design flood extent for the 1-in-100-year plus climate change event. Surface water risk is low: Environment Agency mapping for the 1-in-30, 1-in-100 and 1-in-1000-year events shows the site outside any area of elevated surface water flood risk, with some localised risk on the adjacent roads and footways. Sewer flooding risk is low: the Strategic Flood Risk Assessment records sewer flood incidents in the area, but none on site or in close proximity. Risk from reservoirs and other artificial sources is low, the site lying outside the reservoir flood extent on both a wet day and a dry day. Tidal risk is negligible, the site lying well above tidal influence.

Groundwater risk is high. The FRA bases the rating on the Strategic Flood Risk Assessment's groundwater flood mapping, which places the site in the more than 75% susceptibility class, and records that no site-specific records or historical incidents of groundwater flooding were identified within or adjacent to the site.

Mitigation and Resilient Construction

The FRA gave no flood levels and did not set a finished floor level. Its mitigation is a set of flood resistance and resilience measures. Against fluvial flooding, external ground levels are to be profiled to direct overland flow away from building entrances, floors are to be built in solid concrete with resilient finishes, skirtings, door frames and window sills on the ground floor are to be of resilient materials, and service entry points are to be sealed against water ingress.

Against groundwater, all below-ground structures, including foundations and any retaining walls, are to be built in waterproof concrete with sealants. Groundwater levels are to be monitored during construction, especially during excavation, and sump pumps or positive dewatering are to be deployed if groundwater ingress is observed.

Outcome

The case officer assessed the application against the Environment Agency's standing advice, which asks for estimated flood levels and the finished floor levels of the extension. Because the house and the extension footprint lie outside the Flood Zone 2 outline, the officer took a proportionate approach and accepted the FRA's mitigation details without flood levels. The Royal Borough of Windsor and Maidenhead granted planning permission on 6 August 2026 under reference 26/01427/FULL. Condition 3 requires the development to follow the flood mitigation and protection measures in the FRA and to maintain them for the lifetime of the development.