Flood Risk Assessment — Flood Zone 2 Side Extension, Elm Farm, Dorney, Buckinghamshire
We prepared a site-specific Flood Risk Assessment for a part two-storey side extension at Elm Farm, Boveney Road, Dorney, Buckinghamshire — a Flood Zone 2 site on the north bank of the Thames between Eton and Maidenhead, on behalf of Gareth Cotter of Gareth Cotter Architecture. The extension provides a garden room with a bedroom and ensuite over, together with a reorganised kitchen, dining and utility arrangement, and we worked from drawings 2609-GA-001A and 2609-GA-110 of July 2026. Fluvial, groundwater and artificial-source risks are each medium; tidal, surface water and sewer risks are low. Mitigation sets finished floor levels with freeboard above the modelled design flood level, specifies waterproof concrete with sealed construction joints to below-ground elements, and raises electrical services clear of it. The extension does not reduce floodplain storage or impede conveyance, and the modest increase in impermeable area is accommodated without increasing peak discharge.
Flood Risk Assessment for a Part Two-Storey Side Extension in Flood Zone 2, Elm Farm, Boveney Road, Dorney
Location: Elm Farm, Boveney Road, Dorney, Buckinghamshire, SL4 6QD | Client: Gareth Cotter, Gareth Cotter Architecture | Services: Flood Risk Assessment
A Rural Thames-Side Setting with No Single Governing Source
Dorney sits on the north bank of the Thames between Eton and Maidenhead, on ground that is low-lying, agricultural in character and within the influence of both the river and the Jubilee River flood alleviation channel. Elm Farm occupies a substantial rural plot in that setting.
Rural Thames-side sites of this kind tend to produce a moderate rather than an extreme risk profile: a Flood Zone 2 designation, medium ratings across several sources, and no single dominant constraint. That does not make the assessment easier. It makes the judgement about proportionality more important, because there is no governing source to organise the report around and no obvious single measure that answers everything.
The Site and the Proposals
We were appointed by Gareth Cotter of Gareth Cotter Architecture to prepare a site-specific Flood Risk Assessment in support of the planning application for a part two-storey side extension at the existing detached dwelling at Elm Farm, Boveney Road, Dorney, Buckinghamshire, SL4 6QD. The extension contains a garden room with a bedroom and ensuite over, together with a reorganised kitchen, dining and utility arrangement and associated internal alterations. We worked from drawings 2609-GA-001A and 2609-GA-110, dated July 2026. The site lies within Flood Zone 2 on the Environment Agency Flood Map for Planning.
Evidence Base
We drew on the Environment Agency Flood Map for Planning, the Risk of Flooding from Rivers and the Sea and Surface Water Spatial Planning datasets, the EA reservoir inundation mapping, BGS 1:50,000 geological mapping, EA LiDAR composite terrain data, the recorded sewer flooding history for the postcode, and the Buckinghamshire Strategic Flood Risk Assessment.
Flood Risk Profile
The site was assessed against all six sources.
Tidal flooding is low; the site lies well upstream of the tidal limit.
Fluvial flooding is medium, consistent with the Flood Zone 2 designation — within the 1-in-1000-year extent and outside the 1-in-100-year extent.
Surface water flooding is low, with no significant modelled ponding across the developed part of the plot.
Groundwater flooding is medium, reflecting the permeable superficial deposits of the Thames valley and their hydraulic connection to the river.
Sewer flooding is low, with no incident recorded at the development address.
Flooding from artificial sources is medium, reflecting the modelled inundation extent associated with impounded water upstream.
Development Vulnerability
The extension adds a garden room at ground floor with a bedroom and ensuite above. Introducing sleeping accommodation at first floor rather than ground floor is the appropriate arrangement on a Flood Zone 2 site, and it means the extension does not increase the vulnerability of accommodation at the level where floodwater would arrive.
The proposals constitute a minor householder extension. In accordance with note 62 of the National Planning Practice Guidance, the Sequential and Exception Tests are not required, and the assessment focuses instead on demonstrating that the development is safe for its lifetime and does not increase flood risk elsewhere.
Flood Mitigation and Resilience
Finished floor levels within the extension are set to provide freeboard above the modelled design flood level. Below-ground elements are specified in waterproof concrete with sealed construction joints, addressing the medium groundwater rating. Internal finishes at ground floor use moisture-resistant materials — resilient plaster, low water-absorption insulation and sealed pipework — and electrical services are raised above the design flood level.
The extension does not reduce floodplain storage or impede conveyance, and the modest increase in impermeable area is accommodated within the existing drainage arrangement without increasing peak discharge. The occupants are directed to register with the Environment Agency Floodline warning service.
Outcome
The FRA demonstrated that the proposed development is at an acceptable level of flood risk for its lifetime, and that flood risk at the site and at surrounding sites is not increased as a result of the development. Buckinghamshire Council was provided with an assessment matched in depth to a minor householder scheme on a moderate-risk rural site.