Glossary
Flood risk assessments for sites near London's rivers
Required for development in Environment Agency flood zones 2 or 3, or for basements.
Required for development in Environment Agency flood zones 2 or 3, or for basements.
A flood risk assessment shows how a development is affected by flooding and how it affects flood risk elsewhere. It is required for any site in Flood Zone 2 or 3, for sites over one hectare in Zone 1, and for smaller sites in Zone 1 with known surface water or drainage problems.
The Environment Agency flood map for planning divides land into Zone 1 (low probability, under 1 in 1,000 annual risk), Zone 2 (medium, between 1 in 100 and 1 in 1,000 for rivers), and Zone 3 (high, 1 in 100 or greater for rivers, 1 in 200 for the sea). Zone 3b is functional floodplain.
Large parts of riverside London sit in Zone 3 behind the Thames tidal defences, which is why a householder extension in Hammersmith, Wandsworth or Tower Hamlets can need an assessment when an identical extension two miles inland does not.
The flood sources affecting the site, the design flood level including a climate change allowance, finished floor levels set above it, flood-resilient construction where levels cannot be raised, safe access and escape, and the surface water drainage strategy demonstrating no increase in run-off.
For a householder extension the requirement is usually proportionate: a short assessment with levels, resilience measures and a drainage note, rather than hydraulic modelling.
The sequential test steers development to areas at the lowest flood risk. It does not apply to householder extensions or changes of use to a single dwelling, but it does apply to new dwellings, which is a frequent obstacle for garden development and outbuilding-to-dwelling conversions in riverside boroughs.
Flood risk assessment in a Newham context: London Borough of Newham (Newham Council) runs 10 conservation areas including the Manor Park Conservation Area and the Forest Gate Conservation Area, and an Article 4 direction is in force there, so permitted development cannot be assumed. That decides how flood risk assessment is handled: £548 and 8 weeks for a Newham householder application, or £274 for the Lawful Development Certificate that records flood risk assessment as lawful. The law behind flood risk assessment sits in the Town and Country Planning Act 1990, the GPDO 2015 and the Building Regulations 2010.
| Zone 1 | Low risk, under 1 in 1,000 |
|---|---|
| Zone 2 | Medium, 1 in 100 to 1 in 1,000 |
| Zone 3a | High, 1 in 100 or greater |
| Zone 3b | Functional floodplain |
| Threshold in Zone 1 | 1 hectare, or known drainage issues |
| Sequential test | Not applied to householder extensions |
Only if the site is in Flood Zone 2 or 3, or has recorded surface water flooding.
Almost always in London, because of groundwater and surface water flow paths.
The Environment Agency flood map for planning, by postcode.
Pick a slot that suits you. No obligation and no sales pressure, just straight answers on your planning route.
