How to check flood risk for a development site

Flood risk is one of the first things to check on any UK development site — it shapes what you can build, where, and whether a Flood Risk Assessment will be required. This guide explains the Environment Agency flood zones and how to see them on your own site.

The Environment Agency flood zones

The EA Flood Map for Planning divides land by annual probability of flooding from rivers and the sea:

• Flood Zone 1 — low risk. Less than 0.1% annual probability. Most development is acceptable here.
• Flood Zone 2 — medium risk. Between 0.1% and 1% (rivers) or 0.1% and 0.5% (sea) annual probability.
• Flood Zone 3 — high risk. 1% or greater (rivers) or 0.5% or greater (sea). More vulnerable uses are restricted and a Flood Risk Assessment is typically required.

These zones ignore the presence of flood defences, so a defended site can still sit in Zone 3.

Check flood risk on your site

In Airship Site Designer, search your postcode and turn on the GIS site-data overlays: Flood Zones 2 and 3 are drawn straight onto the aerial photo of your site, alongside flood storage areas. You can see at a glance which part of a site is constrained and lay out homes, roads and open space accordingly. The UK planning constraints map guide covers the other layers available.

What to do next

• Keep more vulnerable development out of Zone 3 where possible; put open space, parking or SuDS there.
• Commission a Flood Risk Assessment early if any part of the site is in Zone 2 or 3.
• Check surface-water and reservoir flood risk separately — the planning flood zones cover rivers and sea only.

The overlays in Airship are indicative screening data drawn from the Environment Agency via planning.data.gov.uk. Always confirm with the official Flood Map for Planning and your local planning authority before relying on it.