Get a flood risk assessment for your development

If your development site could flood, the local planning authority will want to see a flood risk assessment with your planning application. It may refuse the application if the assessment is missing or not good enough. Planning policy steers new building away from land at risk of flooding, and where that is not possible it expects the development to be safe for its whole lifetime without making flooding worse for anyone else.

Find your flood zone

The Environment Agency's Flood Map for Planning shows which flood zone a site is in. The zones describe the chance of flooding from rivers and the sea each year, ignoring flood defences:

• Flood Zone 1, low probability: less than 0.1%
• Flood Zone 2, medium probability: between 1% and 0.1% from rivers, or between 0.5% and 0.1% from the sea
• Flood Zone 3a, high probability: 1% or more from rivers, or 0.5% or more from the sea
• Flood Zone 3b, the functional floodplain, where water has to flow or be stored in a flood

The map also shows land at risk of surface water flooding now or in future, and land that will be at risk from rivers or the sea once climate change is taken into account. Your local planning authority's strategic flood risk assessment identifies the functional floodplain and other local sources of flooding.

Check whether you need an assessment

A site-specific flood risk assessment should go with every development proposal in Flood Zones 2, 3a and 3b, including minor development and changes of use. In Flood Zone 1 you need one for:

• a site of 1 hectare or more
• land the Environment Agency has identified as having critical drainage problems
• land that the Flood Map for Planning or the strategic flood risk assessment shows to be at risk of flooding from any source, now or in future

For a simple, low-risk development, such as a house extension, you may be able to write the assessment yourself. For anything else you will usually pay a flood risk specialist. If you are not sure, ask the Environment Agency.

What the assessment covers

The assessment should match the scale, nature and location of your development. It identifies every source of flooding that could affect the site, applies the Environment Agency's latest climate change allowances, and shows how the development will:

• place its most vulnerable uses on the parts of the site at lowest risk
• stay safe for its lifetime, taking account of who will use it
• manage any remaining risk, with safe access and escape routes where needed
• resist flood water and recover quickly after a flood
• avoid increasing flood risk elsewhere

Assume a residential development will last at least 100 years. For a non-residential development, at least 75 years is the usual starting point.

If your development could affect drainage on or around the site, or is in an area at risk of flooding, include a sustainable drainage strategy, either within the assessment or as a separate document. Sustainable drainage should follow the National Standards for Sustainable Drainage Systems, and for major development the planning authority takes account of advice from the lead local flood authority.

Get flood data from the Environment Agency

Download a copy of the Flood Map for Planning for your site, and request the detailed flood risk assessment map, which shows modelled flood levels, defences and past floods where they exist. The local Environment Agency team sends it within 20 working days. If you rely on an existing flood model, check that it is current, uses the latest climate change allowances and is detailed enough for your site.

Pass the sequential and exception tests

Where the sequential test applies, your application should show that no site at lower risk is reasonably available for the same development within its likely catchment, such as the town or village it would serve. A site can count as reasonably available even if you do not own it. The planning authority decides whether the test is passed.

The sequential test also does not apply to a site at risk only from surface water, where the layout, design and mitigation keep people safe for the lifetime of the development without increasing flood risk elsewhere.

Some uses are not allowed at all in the higher-risk zones. For example, homes and other more vulnerable uses should be refused in the functional floodplain, Flood Zone 3b.

Expect the Environment Agency to be consulted

The planning authority must consult the Environment Agency on applications in certain flood risk areas before it grants permission. If the Agency maintains an objection on flood risk grounds to a major development in Flood Zone 2 or 3, or on land with critical drainage problems, and the authority still wants to approve it, the authority must refer the application to the Secretary of State, who can decide to call it in.

Talking to the planning authority and the Environment Agency before you apply can save you redesigning the scheme later.