Flood Risk Assessment with Floodplain Compensation — Flood Zone 3b Extension, Staines-upon-Thames
We prepared a Flood Risk Assessment with floodplain compensation design for a two-storey remodelling and extension at 92 Thames Side, Staines-upon-Thames — a Flood Zone 3b site in the functional floodplain of the non-tidal River Thames, on behalf of Kathleen Hoy. The extension adds approximately 21.5m² of ground floor footprint on the rear elevation, is More Vulnerable development, and sits where groundwater risk is rated High. The design flood level — the 1% annual probability event including climate change — is 14.91m AOD, and built conventionally the extension would remove 17.8m³ of floodplain storage below it. We set out two compensation routes: a piled floodable void with demolition of the existing conservatory, giving a net gain of approximately 17.8m³; or excavated compensation of 132m² at a mean depth of 0.16m providing 20.8m³, a net gain of approximately 2.7m³. Finished floor level is raised to 15.21m AOD.
Flood Risk Assessment with Floodplain Compensation Design in the Functional Floodplain, 92 Thames Side, Staines-upon-Thames
Location: 92 Thames Side, Staines-upon-Thames, Surrey, TW18 2HF | Client: Kathleen Hoy | Services: Flood Risk Assessment; Floodplain compensation design
The Exceptional Route Through Flood Zone 3b
There is no ordinary route to consent for a residential extension in the functional floodplain. We prepared this Flood Risk Assessment on technical merits under Appendix E of the Spelthorne Flooding SPD (2012), which sets out the exceptional basis on which such a proposal may be found acceptable: it must result in no net loss of flood storage capacity, and no net increase in impedance to the flow of flood water.
Those two requirements are quantitative. They are not satisfied by a statement of intent or by resilient construction, however good. They require a volume calculation tied to a design flood level, and a demonstration that the volume balances.
The Site and the Proposals
We were instructed by Kathleen Hoy to prepare this assessment in support of householder application 25/01183/HOU at 92 Thames Side, Staines-upon-Thames, TW18 2HF. The proposal is a two-storey remodelling and extension of the existing dwelling, adding a new ground floor footprint of approximately 21.5m² on the rear (south) side, with sleeping accommodation at first floor.
The site lies within Flood Zone 3b, the functional floodplain of the non-tidal River Thames, and the proposal is a More Vulnerable use. Groundwater risk across the site is rated High, consistent with the permeable Thames valley gravels beneath this reach and their hydraulic connection to the river.
The Design Flood Level and the Displaced Volume
The design flood level — the 1% annual probability event including a climate change allowance — at the site is 14.91m AOD.
Built conventionally, the extension would remove 17.8m³ of floodplain storage below that level. That figure is the whole basis of the assessment, and everything that follows is about offsetting it. Establishing it required the modelled level for the reach and the footprint and ground levels of the proposed structure below that level, taken from the architect's drawings and the site survey rather than assumed.
Two Routes to Compensation
We presented two options, either of which satisfies the policy test.
Option 1, a floodable void. Build the extension on piles with an open floodable void beneath, so that floodwater continues to occupy the volume the extension would otherwise displace. Combined with the demolition of the existing conservatory, which itself occupies approximately 17.8m³ of floodplain, this produces a net gain of approximately 17.8m³ of floodplain storage.
Option 2, excavated compensation. Excavate compensatory storage at the front of the site: 132m² at a mean depth of 0.16m, providing 20.8m³. Set against the 17.8m³ displaced by the extension, the net effect is a gain of approximately 2.7m³ of floodplain storage.
Both options provide betterment supported by Policy E3(7) — not merely no net loss, but a net improvement. Offering two routes matters practically: it lets the applicant and the authority weigh buildability and cost against each other without reopening the flood risk case.
Level, Resilience and Drainage
Finished floor level is raised to 15.21m AOD, more than 300mm above the design flood level of 14.91m AOD. Sleeping accommodation sits at first floor, well clear of the modelled level, and provides safe refuge in the event that access is lost.
The scheme incorporates flood-resistant and flood-resilient construction, evacuation to that first floor refuge, and a surface water strategy using a green (living) roof to the flat-roof areas together with rainwater planters, reducing runoff rates from the developed footprint. Below-ground elements are specified in waterproof concrete against the High groundwater rating.
The proposal is designed to satisfy each criterion of Policy E3(6)(c) as well as E3(3), and the assessment works through them in turn rather than asserting compliance in the round.
Outcome
On this basis the development is considered to be at an acceptable level of flood risk and does not increase flood risk elsewhere. The commission demonstrates what Flood Zone 3b work actually requires: a design flood level, a displaced volume, a compensation design that balances it, and a floor level set above the water — quantified, and checkable line by line.