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Flood zones and what they mean

Flood zones show areas of land that could flood from rivers (fluvial) or the sea (tidal), ignoring the benefits of any existing flood defences.

They are based on a location's chance of flooding from rivers or the sea in any year.

Flood zones do not take into account:

  • other sources of flooding
  • the possible effects of climate change on rivers or the sea

This means a site in flood zone 1 could still flood now or in the future.

Flood zones combine several datasets to create a picture of flood risk, including:

  • undefended modelling
  • defended modelling
  • recorded flood outlines from past floods
  • other suitable data from third parties

Flood zones take account of flood risk from watercourses with a catchment area greater than 3 square kilometres. They do not have to be classified as main rivers.

Where we have data for watercourses with catchments smaller than 3 square kilometres, we will include it in the flood zones if we consider it suitable.

If a smaller catchment is not included in a flood zone it does not necessarily mean there is no flood risk. If your site is in a smaller catchment not included in a flood zone, you may need further assessment to understand all the risks.

Flood zone 1

Locations in flood zone 1 have a low probability of flooding.

This means, in any year, land has a less than 0.1% (1 in 1,000) chance of flooding from rivers or the sea.

Some flood zone 1 developments need a flood risk assessment as part of their planning application.

Find out when you need a flood risk assessment for development in flood zone 1.

Flood zone 2

Locations in flood zone 2 have a medium probability of flooding.

This means, in any year, land has:

  • between a 1% and 0.1% (between 1 in 100 and 1 in 1,000) chance of flooding from rivers
  • between a 0.5% and 0.1% (between 1 in 200 and 1 in 1,000) chance of flooding from the sea.

Flood zone 2 developments need a flood risk assessment as part of their planning application.

Flood zone 3

Locations in flood zone 3 have a high probability of flooding.

This means, in any year, land has:

  • a 1% (1 in 100) or more chance of flooding from rivers
  • a 0.5% (1 in 200) or more chance of flooding from the sea.

Flood zone 3 developments need a flood risk assessment as part of their planning application.

Flood zone 3b (functional floodplain)

Locations in flood zone 3b are at the highest risk of flooding.

Flood zone 3b is not shown on the flood map for planning service, as it is defined and mapped by local planning authorities (LPAs) in their strategic flood risk assessments.

The difference between flood zones and flood extents

Flood zones are sometimes different to the present day rivers and the sea extents because they combine several additional datasets to create a more complete picture of flood risk.

Flood defences

Flood defences:

  • reduce the probability of flooding from a specific source (a river or the sea)
  • do not completely stop the chance of flooding because there may be a flood that is bigger than the one the defence is designed to protect against (this is called overtopping), or the defence can fail
  • may provide reduced protection over time because of climate change increasing flood risk in the future

There are different types of flood defence. They include:

  • embankments
  • flood gates that are in line with the river
  • flood walls

You can view a complete list of flood defences, called asset categories, on the Defra data services platform.