Flood Risk Assessment — Flood Zone 1 Rear Extension, Harrow
This project was a ground floor rear infill extension at 5 Bishop Ken Road, Harrow, HA3 7HU — a Flood Zone 1 site in the London Borough of Harrow, where the primary flood risk consideration was surface water rather than fluvial flooding. The site is underlain throughout by London Clay Formation, a low-permeability bedrock that rules out infiltration-based drainage, and EA flood mapping identified a modelled surface water flood depth of approximately 0.2m on site, giving an overall medium surface water risk classification. All other flood sources — fluvial, groundwater, sewer, and reservoir breach — were assessed as low risk, with no historic flood events recorded at the address. The surface water strategy specified geocellular attenuation with flow control, profiled external ground levels to intercept overland flows, sealed door thresholds, raised air bricks, and moisture-resistant internal construction throughout. Sequential and Exception Tests were not required given the Flood Zone 1 designation. The assessment confirmed no increase in flood risk to the surrounding area.
Flood Risk Assessment — Ground Floor Rear Extension in Flood Zone 1, Harrow, London
Location: 5 Bishop Ken Road, Harrow, HA3 7HU, London Borough of Harrow | Client: Deepak Khatri | Services: Flood Risk Assessment
When Flood Zone 1 Doesn't Mean No Risk
Most people assume that a Flood Zone 1 site is straightforward from a flood risk perspective — low probability of fluvial flooding, no Sequential or Exception Test required, and a relatively light-touch assessment. That assumption holds for many sites, but it doesn't always tell the whole story. At 5 Bishop Ken Road in Harrow, the fluvial risk is genuinely low, but surface water behaviour in this part of Greater London warranted a closer look.
We were appointed by Deepak Khatri to prepare the FRA in support of a planning application for a ground floor rear infill extension to the existing residential property — a relatively modest proposal, but one located in an area where the West London SFRA flags localised surface water risk and where the underlying geology is not well-suited to natural infiltration.
Site Characteristics and Flood Risk Profile
The site occupies a fully urbanised setting within a traditional residential street in the London Borough of Harrow, bounded by existing dwellings on all sides. There are no main rivers, ordinary watercourses, or open drainage channels in the immediate vicinity. LiDAR analysis confirms relatively uniform ground levels across the site, consistent with the surrounding urban fabric.
The geology is London Clay Formation throughout — a low-permeability bedrock with no superficial deposits recorded at this location. Nearby borehole data from approximately 140 metres east of the site (reference TQ19SE267) recorded clay with high groundwater levels. Soilscapes mapping characterises the soils as slowly permeable, seasonally wet, slightly acid but base-rich loamy and clayey material. In practical terms, this means surface water infiltration is unlikely to be feasible, and runoff management relies entirely on designed drainage infrastructure.
The site falls within Flood Zone 1, confirmed on the EA Flood Map for Planning. Fluvial and tidal flood risk are both low — the site lies well outside the 1-in-100-year plus climate change flood extent, and no EA records of past flood events exist at this location. Groundwater risk is low, with West London SFRA (2018) mapping placing the site outside any area of groundwater susceptibility or elevated groundwater potential, consistent with the low-permeability London Clay Formation. Sewer flooding risk is low, with no incidents recorded at the development address in the West London SFRA data or Thames Water Sewer Flooding History Database (2017), though isolated incidents are noted in the wider area. Reservoir breach risk is low — the site falls outside both the dry-day and wet-day inundation extents.
The one risk source warranting mitigation measures is surface water. EA mapping indicates the site lies within a low to medium surface water flood risk area, with modelled flood depths estimated at approximately 0.2m on site. The West London SFRA (2018) uses an earlier version of the EA risk model, and the more recent NaFRA2 dataset provides a more current picture — both were considered in the assessment. The overall surface water flood risk to the proposed development is assessed as medium.
Surface Water Management Strategy
Given the combination of medium surface water risk, low-permeability clay soils, and an impermeable urban setting, the surface water strategy focused on attenuating runoff rather than infiltrating it. Geocellular attenuation with flow control devices is specified to capture rainwater runoff and discharge at a controlled rate, preventing any increase in surface water flood risk to the surrounding area. External ground profiles are to be designed to direct overland flows away from the extension and toward drainage infrastructure, eliminating local depressions that could accumulate water.
Sealed door thresholds and raised air bricks are specified where practicable. Internal construction throughout is to use moisture-resistant materials: resilient plaster or moisture-resistant plasterboard, low water-absorption insulation, and well-sealed pipework with minimal service voids — all consistent with good practice for a medium surface water risk location.
Outcome
The FRA demonstrated that the ground floor rear extension can proceed in full compliance with NPPF flood risk policy. With the Sequential and Exception Tests not required for Flood Zone 1 development, the assessment focused on evidencing that surface water risk is appropriately managed and that the development introduces no increase in flood risk on or off the site. The London Borough of Harrow was provided with a technically sound, proportionate assessment suited to the scale and nature of the proposals.