Flood Risk Assessment, Flood Zone 3 Roof Extension, West Kensington

Flood Risk Assessment and Exception Test for a Dormer Roof Extension in Flood Zone 3 at West Kensington
Location: 78 Comeragh Road, London, W14 9HR | Services: Flood Risk Assessment

Three High Ratings on One Site

The FRA rated tidal and fluvial, groundwater and sewer flood risk all high at this address. The site lies in Flood Zone 3 and within tidal influence of the Thames. The Hammersmith & Fulham Strategic Flood Risk Assessment (2016) maps it in an area of very high susceptibility to groundwater flooding, and the same SFRA records 418 sewer flood incidents in the W14 postcode between 2006 and 2016.

The works, to a building of four existing flats, comprise a rear roof (dormer) extension, a second-floor rear extension over the back addition, a roof terrace, solar panels and air source heat pumps, front rooflights, and replacement and new doors at lower ground floor.

We were appointed to prepare a site-specific Flood Risk Assessment for a dormer roof extension at 78 Comeragh Road, London, W14 9HR, in the London Borough of Hammersmith & Fulham, in support of the planning application.

Evidence Base

We drew on the Environment Agency Flood Map for Planning, the Risk of Flooding from Surface Water mapping and the reservoir flood maps, BGS geological mapping and a BGS borehole about 80 m from the site, LiDAR topographic data, the Thames Water sewer flooding history mapping, and the Hammersmith & Fulham Strategic Flood Risk Assessment (2016), which supplied the groundwater susceptibility mapping and the sewer flooding record.

Site Characteristics and Flood Risk Profile

The site lies in a predominantly residential area of terraced housing. LiDAR data indicates that ground levels across the site are relatively uniform. BGS mapping shows the London Clay Formation beneath the Kempton Park Gravel, and the borehole about 80 m away records gravel and sand with occasional clay to a depth of 8 to 9 m.

Tidal and fluvial flooding: the site is in Flood Zone 3 on the Flood Map for Planning and within tidal influence, and the FRA rated the risk high. The site lies within the design flood extent for the 1 in 100-year plus climate change event; the development does not increase the building footprint, so it removes no floodplain storage.

Groundwater: the risk is high, from the SFRA groundwater flood mapping. No site-specific records of groundwater flooding were identified within or next to the site.

Sewers: the risk is high. Thames Water sewer flooding history mapping shows properties affected nearby, although not at the development address.

Surface water: the risk is low. Environment Agency mapping places the site outside the identified areas of elevated surface water risk, with localised risk on adjacent roads and footways.

Artificial sources: the risk is medium. The Environment Agency reservoir flood map shows the site partly inside the flood extent from a reservoir in the wet-day scenario.

Development Vulnerability and the Exception Test

The proposals are More Vulnerable development in Flood Zone 3, and the FRA applied the Exception Test. Its grounds were that the roof extension is built above the existing ground floor extension, adds no impermeable area and generates no additional surface water runoff; that the finished floor level of the roof extension is above the extreme water level up to 2100 of 5.92 mAOD; and that a sedum green roof is proposed to the flat roof areas.

Measures Proposed

The FRA proposed a sedum green roof to the flat roof areas. The new accommodation at roof level sits above the 5.92 mAOD extreme water level.

Outcome

Approved by Hammersmith and Fulham Council on 16 October 2026 (P/2026/01356/FUL). The FRA concluded that the development can proceed without being subject to significant flood risk, and that flood risk at the site and surrounding sites is not considered to increase as a result of the development.