Flood Risk Assessment — Flood Zone 3 Roof Extension with Exception Test, 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 | Client: Cameron Jones Planning Ltd | Services: Flood Risk Assessment

Three High Ratings on One Site

It is unusual to see fluvial, groundwater and sewer risk all rated high at a single address. That combination is characteristic of this part of West Kensington: low-lying ground within the wider Thames floodplain, a shallow water table in the underlying deposits, and a Victorian combined sewer network with a documented history of surcharging under intense rainfall.

The design response, however, is shaped by what the development actually is. A dormer roof extension adds habitable accommodation at the top of the building. It places nothing new at ground level, removes no floodplain storage, adds no impermeable area and does not change the number of storeys at risk of inundation. Recognising that early kept the assessment proportionate and directed it to the questions that genuinely mattered.

We were appointed by Cameron Jones Planning Ltd 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 and the Risk of Flooding from Surface Water mapping, the tidal Thames defence and breach information held for this reach, BGS geological mapping and borehole records for the surrounding streets, LiDAR topographic data, the Thames Water sewer flooding record for the postcode sector, and the Hammersmith & Fulham Strategic Flood Risk Assessment, which remains the policy authority for groundwater and sewer flooding in the borough.

Site Characteristics and Flood Risk Profile

The site forms part of a terrace of substantial Victorian mansion-style dwellings. Ground levels across the plot are effectively uniform and consistent with the surrounding street.

Fluvial and tidal flooding is the governing source. The site falls within Flood Zone 3, reflecting residual risk in a defence overtopping or breach scenario on the tidal Thames.

Groundwater risk is high, consistent with the local hydrogeology and recorded water levels in the wider area. Sewer flooding risk is high, reflecting recorded incidents in the surrounding postcode sector on the combined network. Surface water risk is low, with no significant modelled ponding across the plot and no overland flow route crossing the site. Artificial-source risk is medium.

Development Vulnerability and the Exception Test

The proposals are More Vulnerable development in Flood Zone 3, which engages the Exception Test. We set out the case on two limbs.

On wider sustainability benefits, the proposal makes better use of an existing dwelling within an established residential area without extending the building footprint or displacing floodplain.

On safety, the extension increases the amount of accommodation available above the modelled flood level, which improves rather than degrades the safe refuge position for occupants. The development passed the Exception Test on that basis.

Flood Mitigation and Resilience

Mitigation focused on the resilience of the building as a whole and on occupant safety rather than on the new roof space in isolation. The recommendations cover flood-resilient specification at the lower levels of the property: waterproof concrete to below-ground elements, moisture-resistant internal finishes, raised electrical services and sealed service penetrations. Those measures answer the high groundwater and sewer ratings as directly as they answer the fluvial one, because all three deliver water at or near ground level.

Occupant safety is addressed through registration with the Environment Agency Floodline warning service and a defined safe refuge at upper floor level, which the extension itself enlarges.

Outcome

The FRA demonstrated that a roof-level extension in Flood Zone 3 is acceptable in flood risk terms, passes the Exception Test, and does not increase flood risk on or off the site. It is a useful illustration that the flood zone alone does not determine whether a scheme is deliverable — what matters is where in the building the development sits, and what it does to the site's storage, conveyance and refuge position.