Flood Risk Assessment - Flood Zone 3 Office to Residential Change of Use, Guildford Road, Chertsey

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

Chertsey's Flood Setting — Why This Location Demands a Careful Approach
Runnymede is one of Surrey's most flood-exposed boroughs, and Chertsey town centre sits at the heart of that picture. The borough lies within the catchments of both the River Thames and The Bourne, and the EA Flood Map for Planning shows extensive Flood Zone 3 coverage across much of the urban area. The history of flooding here is well-documented — the Runnymede SFRA records repeated riverine incidents going back several decades, and the town has experienced some of the most significant Thames flood events in living memory.

Against that backdrop, converting a previously developed office building to residential use on Guildford Road was never going to be a straightforward planning exercise. The introduction of residential occupation — a more vulnerable land use classification under the NPPF — into a Flood Zone 3 environment requires a credible, evidence-led flood risk case, even where the application benefits from prior approval status. We were appointed by Arkidraft Ltd to prepare the site-specific FRA, addressing all relevant flood mechanisms and demonstrating that the scheme would remain safe throughout its lifetime without increasing flood risk to the surrounding area.

Understanding the Site and Its Flood Risk Profile
The site at 2 Guildford Road is a previously developed brownfield plot, currently occupied by an office building and surrounded by a predominantly residential neighbourhood of traditional housing. It is well-integrated into the existing urban grain of Chertsey and represents a logical candidate for residential conversion — the technical challenge lies in managing the flood risk that comes with its location.

Bedrock geology at the site comprises Bagshot Formation Sand, overlain by Kempton Park Gravel Member superficial deposits — primarily sand and gravel with localised clay lenses. Borehole records from approximately 180 metres to the south of the site (reference TQ06NW153) indicate groundwater encountered at around 2m below ground level. Soilscapes mapping characterises the soils as loamy with naturally elevated groundwater conditions, and these characteristics limit the feasibility of infiltration-based drainage at this location. Surface water generated on site needs to be actively managed through designed attenuation rather than discharged to ground.

The site is partially within Flood Zone 3, confirmed on the EA Flood Map for Planning, and lies within the modelled design flood extent for the 1-in-100-year plus climate change event. There are no formal flood defence structures providing protection at this location. Environment Agency Product 4 hydraulic modelling data was requested to establish the definitive design flood level, with proposed finished floor levels to be set at a minimum of 300mm above that figure — consistent with current planning practice guidance and ensuring the residential use is demonstrably more flood resilient than the office use it replaces.

Surface water flood risk adds a further layer of complexity. EA pluvial flood mapping and the Runnymede SFRA (2018) both identify a high-risk surface water flow path affecting the rear portion of the site. This combination of fluvial and surface water flood mechanisms is relatively uncommon in urban conversion projects and required each source to be addressed through a distinct strand of the mitigation strategy.
Groundwater susceptibility, per the Runnymede SFRA (2018), is limited to below-ground structures. As the development proposes no basement construction whatsoever, groundwater risk to the end use is negligible and no specific groundwater mitigation is required. Sewer flooding history is minimal — the SFRA records only a small number of external incidents in the wider Chertsey locality, with no internal property incidents at or adjacent to the site. Reservoir breach mapping confirms the site lies outside both the dry-day and wet-day inundation extents, giving a low risk classification for artificial sources.

A Two-Strand Mitigation Strategy
Because the site faces meaningful risk from two distinct flood mechanisms — fluvial inundation and surface water overland flow — the mitigation strategy was structured to address each one separately rather than relying on a single catch-all approach.

For fluvial risk, the principal measure is the specification of finished floor levels at a minimum of 300mm above the 1-in-100-year plus climate change design flood level derived from EA Product 4 data. Ground floor construction will use solid concrete slabs with moisture-resistant finishes throughout. Skirtings, door linings, and window components are to be specified in resilient materials capable of withstanding inundation and drying quickly without structural failure. All service penetrations through external walls and floor slabs will be sealed to prevent water ingress. External ground levels will be profiled to direct overland flow away from principal access points and building entrances. Since the development does not increase the existing building footprint, there is no reduction in floodplain storage capacity and no increase in flood risk to neighbouring properties or the wider catchment.

For surface water risk, the drainage strategy incorporates geocellular attenuation systems with controlled discharge, limiting post-development runoff rates to the existing greenfield equivalent where applicable, or at minimum ensuring no increase in surface water volumes reaching the surrounding drainage network. Raised thresholds at all ground floor door openings, sealed external openings, and moisture-resistant internal finishes complete the surface water resilience package. The combination of these measures means the development is well-placed to manage even the more significant surface water events modelled for the site.

A Note on Planning Status
As a prior approval change of use application, this development is exempt from the Sequential and Exception Tests under NPPF note 62. That exemption does not, however, diminish the importance of the technical flood risk case — Runnymede Borough Council still needs to be satisfied that the scheme is safe and will not increase flood risk elsewhere. The FRA provided that technical evidence base: a proportionate but thorough assessment of all relevant flood mechanisms, with a clearly specified and deliverable mitigation strategy to match.

Outcome
The assessment demonstrated that the proposed change of use from office to residential satisfies national flood risk policy requirements in full, subject to implementation of the specified floor levels and resilience measures. Runnymede Borough Council was provided with a technically robust document addressing the combined fluvial and surface water flood characteristics of this part of Chertsey, giving the authority the evidence it needed to determine the prior approval application with confidence.