Flood Risk Assessment — Flood Zone 3 Office-to-Residential Conversion, Mill Place, Kingston upon Thames

Flood Risk Assessment — Office-to-Residential Conversion in Flood Zone 3, Kingston upon Thames
Location: 40 Mill Place, Kingston upon Thames, KT1 2RJ, Royal Borough of Kingston upon Thames | Client: Gerry Murphy | Services: Flood Risk Assessment
Converting in Flood Zone 3 — A Different Kind of Challenge

Office-to-residential conversions in Flood Zone 3 present a particular planning challenge. The change of use introduces a more vulnerable land use into a high-risk environment, and whilst prior approval applications are exempt from the Sequential and Exception Tests under NPPF note 62, the flood risk case still needs to stack up. The site at 40 Mill Place sits squarely within this context — a vacant office block in a predominantly residential part of Kingston upon Thames, within Flood Zone 3 of the Hogsmill catchment, and subject to high fluvial flood risk from that watercourse as well as medium risk from reservoir breach.

We were appointed to prepare the FRA and flood resilience strategy in support of the prior approval application.

Site and Flood Risk Context
The site occupies previously developed brownfield land within a fully urbanised setting, bounded by residential properties on three sides with a vehicle repair shop to the south. Geology comprises London Clay Formation bedrock — a low-permeability material — overlain by Langley Silt Member superficial deposits. Nearby borehole data from approximately 90 metres east of the site recorded dry conditions in trial pits, consistent with the low groundwater susceptibility recorded in the Kingston Strategic Flood Risk Assessment (SFRA, 2021), which places the site within the less than 25% groundwater flood susceptibility category.

Fluvial flood risk from the Hogsmill River is high. Environment Agency Product 4 hydraulic modelling data — derived from the Hogsmill 2015 model — confirmed a 1-in-100-year plus 35% climate change flood level of 9.79m AOD at the site. The existing building had an internal finished floor level of 9.87m AOD, sitting just 80mm above the design flood level and providing only marginal inherent resilience under current conditions.

Surface water flood risk across the site itself is low, supported by flat topography and established road drainage infrastructure intercepting overland flows. Sewer flooding risk is low, with 34 to 97 incidents recorded across the wider Kingston area per the SFRA, none of which directly affect the site. Reservoir breach risk is medium, with EA mapping confirming the site falls within the wet-day inundation extent associated with upstream reservoir failure.

Flood Mitigation Strategy
The mitigation approach centred on raising the internal finished floor level to 10.09m AOD — providing 300mm of freeboard above the 1-in-100-year plus 35% climate change design flood level, exceeding the minimum typically required under current planning practice guidance. This makes the proposed residential use demonstrably more flood resilient than the existing office building it replaces.

Ground floor construction was specified throughout in flood-resilient materials: resilient skirtings, doorframes, and window sills; sealed service entry points against water ingress; and reinforced double-glazed frames for doors and windows to resist potential high-energy flows. Internal finishes and insulation were specified to be moisture-tolerant to facilitate rapid recovery following any inundation event.
Since the development does not increase the building footprint, no reduction in floodplain storage results from the proposals, and no increase in flood risk to neighbouring properties or the wider catchment is anticipated.

Emergency Preparedness
A Flood Response Plan for the site was originally prepared in 2008 and remains in place. The plan sets out established emergency procedures for occupant evacuation, including designated pedestrian and vehicular evacuation routes, and provides guidance on pre-flood preparation and post-flood re-occupation. Future occupants are advised to register with the Environment Agency's Floodline Warnings Direct service to receive advance notification of flood events affecting the Hogsmill catchment.

Outcome
The FRA demonstrated that the proposed development, with the specified floor level and resilience measures in place, can proceed safely in accordance with NPPF flood risk policy. The Royal Borough of Kingston upon Thames was provided with a clear, technically robust evidence base to support the prior approval application — demonstrating that a long-vacant office building could be brought back into productive residential use without increasing flood risk on or off the site.