Case Study: Kitchen Extension in Flood Zone 3 Behind the River Aire Embankment — Householder FRA, Kellington

Written by Antony Riozzi BEng (Hons) — Director & Principal Consultant, Flood Risk Environmental. 17+ years in flood risk & drainage, with experience at a Lead Local Flood Authority, a national housebuilder and in consultancy. About the author →

Location

Kellington, near Goole, North Yorkshire

Development

13.8 m² single-storey wraparound kitchen extension to a detached house

Authorities

North Yorkshire Council (LPA and LLFA) · Environment Agency data

What we delivered

Householder FRA with Environment Agency modelled levels · design flood level and sensitivity · flood resistance and resilience specification · floodplain displacement check · soakaway drainage drawing

Outcome

Extension shown to be safe for its lifetime with no increase in flood risk elsewhere; FRA submitted with the householder application

The challenge

The house sits on the southern edge of the River Aire floodplain, about 100 m south of the raised embankment that keeps the village dry, inside the Environment Agency's flood warning area. The flood map puts it in Flood Zone 3. For a householder extension the sequential and exception tests do not apply and the Environment Agency's standing advice for minor extensions does — but the council still needs an FRA that shows the extension is safe and does not increase flood risk elsewhere. The extension floor has to match the existing house, so it cannot be raised; the mitigation has to come from the construction.

FRE plan of the Environment Agency Flood Map for Planning around the Kellington site showing Flood Zone 3, the climate change extent and the River Aire embankment — Flood Map for Planning around the site from our open-data plan series: Flood Zone 3 (present day), Flood Zone 2, the climate-change extent and the River Aire flood defence to the north. Flood data © Environment Agency (OGL); basemap © MapTiler © OpenStreetMap contributors.

What we did

LiDAR terrain map of the Kellington site and surroundings showing the low-lying property relative to the raised River Aire embankment to the north — Ground levels around the site from the Environment Agency LiDAR terrain model. The embankment to the north stands at 7.87–7.89 mAOD; the extension sits at about 6.70 mAOD on the floodplain side of the village. {LIDAR}

Open flood maps do not give levels, and levels are what this assessment turns on, so we requested the Environment Agency's flood risk assessment data pack for the site. It came from the 2017 Lower Aire study and gave modelled flood levels at a point within the footprint of the house for a range of events with and without the embankment.

With the embankment in place the site does not flood in any present-day event up to the 1 in 200 year; it floods only in the 1 in 1,000 year event, to 8.16 mAOD. With climate change, the defended 1 in 100 year event reaches the site at 7.70, 7.86 and 8.05 mAOD for the +20%, +30% and +50% peak flow runs. Applying the Environment Agency's guidance for a More Vulnerable use in Flood Zone 3a, we adopted the +30% run as the design case — a design flood level of 7.86 mAOD, 1.16 m above ground and 0.86 m above the finished floor — and tested the upper-end +51% allowance as a sensitivity.

Because the floor cannot be raised, the extension is designed to resist and then recover. Flood resistance to 600 mm above the finished floor — a certified flood doorset and self-closing airbricks — keeps water out of the more frequent events; flood resilient construction of the whole extension up to 8.16 mAOD, the design level plus 300 mm, means that if water does get in the fabric dries out rather than being replaced. There is no ground-floor sleeping accommodation and the first floor is a refuge above even the 1 in 1,000 year plus climate change level.

FRE drainage drawing for the Kellington extension showing the roof water drained to a new soakaway positioned clear of the building and boundaries — FRE drawing — surface water drainage for the extension. Roof water goes to a new soakaway sized on the new roof area, set at least 5 m from the building and 2.5 m from the boundaries, so no additional runoff leaves the site.

The other question the council asks is whether the extension makes flooding worse for anyone else. We calculated the floodplain volume the new footprint displaces in the design event — 16.0 m³ — and set it against the modelled flood compartment behind the embankment of some 190 ha. The effect is below the resolution of the Environment Agency's model, and its standing advice for minor extensions does not require compensatory storage. Roof water from the extension drains to a new soakaway so that no runoff leaves the site.

The outcome

A householder FRA that answers the two questions the council and the Environment Agency actually ask — is it safe for its lifetime, and does it increase risk elsewhere — with modelled levels, a design event chosen and justified, a resistance and resilience specification the builder can price, and a drainage drawing, all for a fixed fee. The application went in complete.

If you are extending a house in Flood Zone 2 or 3. Even a small extension needs an FRA the council can rely on, and in a defended area that means obtaining the Environment Agency's modelled levels rather than working from the map. It is usually a fixed fee and a couple of weeks, and it avoids the refusal or the 'further information' letter that stalls so many householder applications.

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Frequently Asked Questions

Do I need a flood risk assessment for a small extension in Flood Zone 3?

Yes. Any development in Flood Zone 2 or 3 needs a site-specific FRA, however small. For a householder extension the sequential and exception tests do not apply and the Environment Agency's standing advice for minor extensions sets what the FRA has to show — that the extension is safe for its lifetime and does not increase flood risk elsewhere.

What if the extension floor cannot be raised above the flood level?

That is normal for an extension, because it has to match the existing house. The mitigation then comes from the construction: flood resistance — a certified flood doorset and self-closing airbricks to 600 mm above the floor — and flood resilient materials up to the design flood level plus freeboard, so the extension recovers quickly if water does get in.

Why request Environment Agency data when the flood map is free?

The flood map shows extents, not levels, and in a defended area it cannot tell you whether the site floods with the embankment in place. The Environment Agency's data pack gives modelled levels at the site for each event, with and without defences and with climate change, which is what the design flood level and the resilience specification are built from.

Does a 13.8 m² extension need floodplain compensation?

No. The volume it displaces — here 16 m³ within a 190 ha flood compartment — is below the resolution of the Environment Agency's model, and the standing advice for minor extensions does not require compensatory storage. The FRA still has to show the calculation.

Kitchen Extension Behind a River Flood Embankment — Householder FRA in Flood Zone 3