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Flood risk assessment · London Borough of Merton

Flood risk assessment in Merton, from £375 fixed

Merton's 2024 Local Plan needs a flood risk assessment for every proposal in Flood Zones 2 and 3, with current climate change allowances. Near the Wandle and Beverley Brook it also keeps buildings 8m back from a main river and 5m from smaller watercourses, and rules out bedrooms at basement level in Flood Zone 3a.

Flood risk assessmentfrom £375
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A red clad outbuilding with a white window standing in brown floodwater among bare trees
Flood-resistant materials are specified to a height above the estimated flood level. Photo: Serge Lavoie / Pexels.

What Merton Council asks of a flood risk assessment

From the council's Merton Local Plan 2024, Policy F15.8 (Managing Local Flooding) (adopted 20 November 2024).

  • The table in Policy F15.8 says a flood risk assessment is needed for all development proposals in Flood Zones 2, 3a and 3b, and a drainage strategy for all new dwellings and major developments. (source, checked 29 October 2026)
  • Developments must use the latest climate change allowances in the flood risk assessment or surface water drainage strategy. (source, checked 29 October 2026)
  • All buildings should be set back from the top of the bank by at least 8m for main rivers and 5m for ordinary watercourses, to allow for maintenance and future flood defence work. (source, checked 29 October 2026)
  • In Flood Zone 3a, self-contained residential basements and bedrooms at basement level will not be allowed; in Flood Zone 2 they will not be allowed without the right mitigation in line with Environment Agency guidance. (source, checked 29 October 2026)
  • In Flood Zone 3b, conversions must not increase the building footprint or the number of units or bedrooms, and changes to a more vulnerable use will not be allowed. (source, checked 29 October 2026)

What Merton flood risk assessment drawings have to show

For a Merton house near the Wandle, the Pyl Brook or the Beverley Brook, the first thing we measure is the distance from the top of the bank. An extension that comes within 8m of a main river, or 5m of an ordinary watercourse, conflicts with Policy F15.8, so we show the set-back line on the site plan.

The assessment then gives the flood level with the current climate change allowance and the floor level above it. If the scheme includes a basement room, we confirm that it is not a bedroom or a separate flat before the drawings go further.

PS2 · at a glance

Merton: 759 householder decisions

Merton
Approvals, prior approvals included92.9%

705 of 759 granted. London: 85.8%.

Decided within 8 weeks74.6%

303 of 406 applications, prior approvals excluded.

Source: MHCLG PS2, 1 July 2025 to 30 June 2026. Dark tick: London.
Two hands holding a printed floor plan over a desk of drawings, with a pen and notebook beside it
Checking the drawings with you before anything goes to the council. Photo: Tima Miroshnichenko / Pexels.

What it costs in Merton

  • Flood risk assessment: from £375.

Flood risk assessment across London · Merton planning rules · every price

Flood risk assessment · Merton

What happens next in Merton

We check your address on every map the council uses, take the flood level with climate change, set the floor level and resilience measures, and mark them on the drawings.

Photo: Helena Jankovičová Kováčová / Pexels

Answers

Merton flood risk assessment questions

Does every extension in a Merton flood zone need a flood risk assessment?
Yes. The table in Policy F15.8 of the Merton Local Plan 2024 says an FRA is needed for all development proposals in Flood Zones 2, 3a and 3b. For a householder extension of up to 250 square metres, it follows the Environment Agency standing advice for minor extensions.
How close to a river can I build in Merton?
Policy F15.8 says all buildings should be set back from the top of the bank by at least 8m for main rivers and 5m for ordinary watercourses. The space is kept for maintenance, future flood defence work and the river corridor, so we mark the line on the site plan before the extension is drawn.
Can I have a basement bedroom in a Merton flood zone?
Not in Flood Zone 3a, where the Local Plan does not allow self-contained basement homes or bedrooms at basement level. In Flood Zone 2 they are not allowed without the right mitigation measures in line with Environment Agency guidance and the Local Plan. Merton basements also need their own impact assessment and drainage strategy.

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