Flood risk assessments for planning

Anyone applying for planning permission may be asked about flood risk.

For some sites, the council expects a flood risk assessment with the application. This page covers the rules in England, with notes on Wales and Scotland at the end.

The flood zones

The Environment Agency's Flood Map for Planning shows which flood zone a site is in. The zones describe the chance of river or sea flooding in any one year.

Zone 1 has less than a 0.1% chance of river or sea flooding each year.
Zone 2 has between a 1% and 0.1% chance of river flooding, or between 0.5% and 0.1% for the sea.
Zone 3a has a 1% or greater chance of river flooding, or 0.5% or greater for the sea.
Zone 3b is the functional floodplain, where water has to flow or be stored in a flood.

The map does not mark Zone 3b separately. Councils identify it in their strategic flood risk assessment.

When an assessment is needed

GOV.UK says to complete one for all development, including minor development and changes of use, in these places.

In Flood Zone 2 or 3.
In Flood Zone 1 on a site of 1 hectare or more.
In Flood Zone 1 where the map shows a risk of surface water flooding, now or in future.
Where the map or the council's assessment shows the risk from rivers or the sea will grow over the life of the development.

The council may refuse an application that has no assessment, or one that falls short. Most applicants pay a flood risk specialist. For a simple, low-risk project such as a house extension, you may be able to write it yourself.

The sequential and exception tests

Planning authorities use the sequential test to steer new building towards the areas least likely to flood. Where a site is at risk, the test compares other "reasonably available" sites at lower risk. Those other sites do not have to belong to the applicant.

The exception test applies when no lower-risk site is available. The development has to bring wider benefits to the community that outweigh the flood risk. It also has to be safe for its lifetime without raising flood risk elsewhere. Changes of use are not normally subject to either test.

Environment Agency data

For all development, GOV.UK says to get Product 1, a copy of the Flood Map for Planning for your site. You should also request Product 4, a detailed flood risk assessment map. The local Environment Agency team sends Product 4 within 20 working days.

If the assessment will use the agency's own hydraulic modelling, you are likely to need Products 5, 6 and 7 as well. Those come from the local Environment Agency office.

When flood modelling comes in

Detailed modelling is more likely for new housing or other large schemes where flood risk is high. It is also more likely where a development could raise the risk of flooding for its neighbours.

The applicant is responsible for checking that any existing model suits the site. The Environment Agency says its own models are not designed to assess other people's developments. It gives a free preliminary opinion on a proposal, and it also sells modelling advice before you apply. For surface water or groundwater modelling, the lead local flood authority advises.

Wales and Scotland

In Wales, Natural Resources Wales publishes its own Flood Map for Planning, which supports the national planning policy TAN 15. A site-specific study there is called a flood consequences assessment.

In Scotland, the Scottish Environment Protection Agency (SEPA) advises planning authorities. They consult it on proposals that meet its thresholds. For everything else, SEPA provides standing advice.