Flood Risk Assessment — Flood Zone 1 Residential Development, Harrow

Flood Risk Assessment — Rear Infill Extension in Flood Zone 1 with High Surface Water Risk, Harrow
Location: 196 Exeter Road, Harrow, HA2 9PH, London Borough of Harrow | Client: Deepak Khatri | Services: Flood Risk Assessment

When Flood Zone 1 Doesn't Tell the Whole Story

A Flood Zone 1 classification can give a false sense of security. At 196 Exeter Road in Harrow, the EA's Flood Map for Planning confirms the site sits comfortably outside any fluvial flood zone — but the surface water flood mapping tells a very different story. The rear garden falls within the high-risk surface water flood extent, and the proposed infill extension to a previously approved rear extension needed a technically robust case to demonstrate it wouldn't exacerbate that risk.

We were appointed by Deepak Khatri to prepare the FRA for the London Borough of Harrow. The development — an infill extension to a previously approved rear extension (reference P/1789/23) — was modest in scale, but the pluvial risk classification meant the assessment needed to go beyond a standard Flood Zone 1 submission.

Site Context and Ground Conditions

The site sits within a predominantly residential area of Harrow, comprising previously developed brownfield land currently occupied by a semi-detached residential property. It is bounded by existing dwellings on all sides. LiDAR data confirms relatively uniform ground levels across the plot, consistent with the surrounding urban landscape.

The underlying geology is London Clay Formation — a widespread, low-permeability bedrock across much of Greater London — with no superficial deposits recorded. A nearby BGS borehole (reference TQ18NW124, approximately 550 metres northwest) confirmed silty clay. Soilscapes mapping characterises the soils as slowly permeable, seasonally wet, slightly acid but base-rich loamy and clayey soils — a profile that limits infiltration capacity and contributes to surface water accumulation during intense rainfall events.

The nearest open water feature is Yeading Brook, approximately 400 metres northwest. No main rivers, ordinary watercourses, or open drainage channels adjoin the site, and DEFRA Magic Map confirmed no designated sites of hydrological significance nearby.

Flood Risk Assessment — Source by Source

Fluvial risk was classified as low. The site lies within Flood Zone 1 with an annual probability of flooding from rivers of less than 0.1%, and falls entirely outside the design flood extent for the 1-in-100-year plus climate change event.

Tidal risk was assessed as negligible. The site is elevated well above tidal influence with no hydraulic connection to the tidal Thames.

Pluvial (surface water) risk was the critical finding — classified as high. EA surface water flood mapping for the 1-in-30, 1-in-100, and 1-in-1000-year events showed the rear of the property falling within high, medium, and low risk areas of elevated surface water flood risk. The rear garden in particular sits within the high-risk pluvial extent.

However — and this was the key technical argument — the footprint of the proposed infill extension does not encroach onto the 1-in-30-year surface water flood risk area (equivalent to Flood Zone 3b). The extension sits on relatively flat ground bordered by road gullies and established drainage infrastructure that intercept surface flows before they reach the building footprint. On that basis, the development would not increase flood risk on-site or elsewhere.

Groundwater risk was classified as low. The underlying London Clay Formation is a low-permeability bedrock that limits groundwater emergence, and the West London SFRA (2018) mapping confirmed the site lies outside areas susceptible to groundwater flooding. No historic groundwater incidents were recorded.

Sewer flooding risk was assessed as low, though the wider context warranted mention. The West London SFRA recorded 21–40 sewer flood incidents in the HA2 postcode since 2000, and Thames Water's Sewer Flooding History Database confirmed a number of affected properties in the area — though not at the development address itself.

Artificial source risk was low. EA reservoir breach mapping confirmed the site lies outside both dry-day and wet-day reservoir flood extents.

No historic flooding was recorded at the site or in its immediate vicinity, consistent with the Flood Zone 1 classification.

Flood Mitigation and Resilience

The mitigation strategy focused squarely on the pluvial risk — the only elevated flood source. Measures included sealed thresholds for all doors and windows, raised air bricks where feasible, moisture-resistant building materials (resilient plaster, moisture-resistant plasterboard, insulation with low water absorption), well-sealed pipework with minimal service voids, and sealed utility conduits.

Sequential and Exception Tests were not required given the Flood Zone 1 designation.

Outcome

The FRA demonstrated full NPPF compliance, confirming that the infill extension at 196 Exeter Road can proceed without increasing flood risk to the site or surrounding properties. The assessment provided the London Borough of Harrow with the technical evidence to support the planning application — establishing that even where surface water risk is high, a carefully positioned extension that avoids the 1-in-30-year pluvial extent and incorporates proportionate resilience measures presents an acceptable risk profile.