Flood Risk Assessment — Flood Zone 3a Residential Extension, Windsor
We prepared a Flood Risk Assessment for a residential extension in Flood Zone 3a at 121 Arthur Road, Windsor — a high fluvial risk site requiring a comprehensive resilience specification to satisfy the LPA and demonstrate no increase in flood risk on or off-site.
Flood Risk Assessment — Shopfront and Glazed Roof Extension in Flood Zone 3a, Windsor
Location: 121 Arthur Road, Windsor, SL4 1RU, Royal Borough of Windsor and Maidenhead | Client: Cameron Jones Planning | Services: Flood Risk Assessment
A Commercial Extension in the Thames Floodplain
Flood Zone 3a doesn't leave much room for ambiguity. At 121 Arthur Road in Windsor, the Environment Agency's Flood Map for Planning places the site squarely within the high-probability fluvial flood zone — land with a 1% or greater annual probability of flooding from rivers. The proposed development — increased glazing area for an existing shopfront and a new glazed roof to the rear — was modest in scale, but the Flood Zone 3a classification meant the FRA needed to be technically watertight.
We were appointed by Cameron Jones Planning to prepare the FRA for the Royal Borough of Windsor and Maidenhead.
Site Context and Geology
The site sits within a predominantly residential area of Windsor, comprising previously developed brownfield land currently occupied by a commercial building. It is bounded by existing dwellings to the west and south and commercial units to the north and east. LiDAR data confirms ground levels are relatively uniform across the plot.
The underlying geology comprises Lambeth Group bedrock — predominantly clay, silt and sand — with Shepperton Gravel Member recorded as superficial deposits consisting of sand and gravel. A nearby BGS borehole record (reference SU97NE12, approximately 320 metres southeast) confirmed gravel, clay and sand. Soilscapes mapping characterises the site as freely draining, slightly acid but base-rich soils, suggesting surface water infiltration may be feasible.
The site is in close proximity to the River Thames, with no designated sites of hydrological significance (SSSIs or flood storage reservoirs) identified nearby on DEFRA's Magic Map.
Flood Risk Assessment — All Sources
Fluvial risk was classified as high. The site lies within the modelled design flood extent for the 1-in-100-year plus climate change event, and EA mapping confirmed that no formal flood defences protect the site. However, the development does not increase the existing building footprint — meaning no encroachment on floodplain storage and no increase in flood risk elsewhere.
Tidal risk was assessed as negligible. Pluvial (surface water) risk was classified as low. Groundwater risk was assessed as medium — the RBWM SFRA (2007) places the site within the 50–75% groundwater susceptibility band. Sewer flooding risk was low. Artificial source risk was medium based on EA wet-day reservoir breach mapping.
Sequential and Exception Tests were not applicable given the non-residential use with no increase in footprint.
Flood Mitigation and Resilience Specification
Double-glazed windows and reinforced frames for all doors and windows exposed to potential high-energy flows; resilient materials for skirtings, doorframes and window sills throughout the ground floor; sealed thresholds with raised air bricks; moisture-resistant building materials including resilient plaster, moisture-resistant plasterboard, and insulation with low water absorption; well-sealed pipework with minimal service voids; and sealed utility conduits.
Outcome
The FRA demonstrated full NPPF compliance, confirming that the proposed glazing works at 121 Arthur Road can proceed without increasing flood risk to the site or surrounding properties. The assessment provided the Royal Borough of Windsor and Maidenhead with the technical evidence needed to support the planning application.