Flood Risk Assessment — Flood Zone 3 Office-to-Residential Conversion, Guildford Road, Chertsey

Flood Risk Assessment — Office-to-Residential Conversion in Flood Zone 3, Chertsey, Surrey
Location: 2a Guildford Road, Chertsey, KT16 9BJ, Runnymede Borough Council | Client: Ali Uddin, Arkidraft Ltd | Services: Flood Risk Assessment

Chertsey's Flood Context — Why This One Required Careful Handling
Chertsey sits within one of the most flood-affected parts of Surrey. The Runnymede Borough straddles the River Thames and The Bourne, and significant areas of Chertsey town centre have well-documented histories of fluvial inundation going back decades. The SFRA historic flooding records confirm repeated incidents in and around the town between 1980 and 2000, and EA flood mapping shows that much of the built environment sits squarely within Flood Zone 3.

2a Guildford Road is a previously developed brownfield site — a sizeable office building set within a predominantly residential neighbourhood of traditional semi-detached housing — and the proposal to convert it to residential studio flats meant introducing a more vulnerable land use into that environment. We were appointed by Arkidraft Ltd to prepare the FRA in support of the prior approval application, demonstrating that the development could proceed safely in line with NPPF flood risk policy.

Understanding the Site's Flood Risk Profile
The geology beneath the site consists of Bagshot Formation Sand bedrock overlain by Kempton Park Gravel Member superficial deposits — predominantly sand and gravel, with occasional clay. Nearby borehole data from approximately 180 metres south of the site recorded groundwater at 2m depth. Soilscapes mapping characterises the soils as loamy with naturally high groundwater, suggesting that infiltration-based drainage is unlikely to be viable at this location.

Fluvial flood risk is high. The site falls partially within Flood Zone 3 as confirmed on the EA Flood Map for Planning, with EA mapping also confirming the site lies within the design flood extent for the 1-in-100-year plus climate change event. No formal flood defences provide protection at this location. Environment Agency Product 4 hydraulic modelling data was requested to confirm the precise design flood level, with the proposed finished floor level to be set at least 300mm above that figure in accordance with NPPF planning practice guidance.

Surface water flood risk adds a further dimension here. EA mapping indicates that the rear portion of the site falls within a high surface water risk area — a finding consistent with the Runnymede SFRA (2018). This is a combination of flood risk sources less commonly encountered in urban conversion projects, and it shaped the resilience strategy accordingly.

Groundwater risk is low. The Runnymede SFRA (2018) places the site within the category of potential groundwater flooding of below-ground property, and as the development proposes no basement construction, the risk to the proposed use is negligible. Sewer flooding risk is also low, with between one and four external incidents recorded in the wider area over the past decade and no internal incidents recorded at all. Reservoir breach risk is low — EA mapping confirms the site lies outside both the dry-day and wet-day inundation extents.

Resilience Strategy
Given both the fluvial and surface water risk profile, the mitigation strategy addressed each source separately. For fluvial risk, finished floor levels are to be set at a minimum of 300mm above the 1-in-100-year plus climate change design flood level, to be confirmed against EA Product 4 data. Ground floor construction throughout is to use solid concrete with moisture-resistant finishes; skirtings, doorframes, and window sills specified in resilient materials; and service entry points sealed against water ingress. External ground profiles are to be shaped to direct overland flows away from building entrances.

For surface water risk, the strategy incorporated geocellular attenuation with flow control devices to manage runoff from the site, sealed door and window thresholds, raised air bricks where practicable, and moisture-resistant plasterboard and low-absorption insulation throughout. As the development does not increase the building footprint, floodplain storage is not reduced, and no increase in flood risk to neighbouring properties or the wider catchment is anticipated.

Outcome
The FRA demonstrated that the proposed change of use to residential studio flats can proceed in accordance with NPPF flood risk policy, subject to the specified floor level and resilience measures being implemented. As a change of use prior approval application, the development is exempt from the Sequential and Exception Tests under NPPF note 62, but the assessment provided Runnymede Borough Council with a thorough, evidence-led technical justification for the planning decision — addressing both the fluvial and surface water risk sources that characterise this part of Chertsey.