Do I Need a Flood Risk Assessment for a House Extension?

When Is an FRA Required for an Extension?

A Flood Risk Assessment is required for a house extension in the following circumstances:

• Your property is in Flood Zone 2 or Flood Zone 3 (medium or high flood risk)
• Your LPA has imposed a site-specific flood risk requirement on your planning application
• The extension would create a new habitable room below ground level (basement conversion) in a flood risk area
• The Environment Agency or your LLFA has raised flood risk as a concern during pre-application discussions

How Do I Know If I'm in a Flood Zone?

The Environment Agency's Flood Map for Planning shows the flood zone classification for all land in England. You can search by postcode or address. Wales uses a different system — the Development Advice Map (DAM) under TAN15 — which you can access via the Natural Resources Wales flood map.

If your property shows as Flood Zone 2 or 3, your local planning authority will almost certainly require an FRA before approving your extension. If you're in Flood Zone 1, check with your LPA before assuming you don't need one.

What Does an FRA for a House Extension Include?

A Flood Risk Assessment for a residential extension in Flood Zone 2 or 3 will typically include:

• Assessment of all flood sources affecting the property (river, sea, surface water, groundwater, sewer)
• EA flood map interrogation and, where available, detailed model data for flood levels
• Climate change assessment for the lifetime of the development
• Recommendations for finished floor levels if the extension creates new habitable space
• Assessment of whether the extension would affect flood risk to the existing property or neighbouring properties
• Confirmation that the extension satisfies NPPF and PPG requirements

How Much Does It Cost & How Long Does It Take?

A Flood Risk Assessment for a house extension typically starts from £295+VAT for Flood Zone 1 sites and from £395+VAT for Flood Zone 2 or 3 properties. We deliver most extension FRAs within 3–5 working days. See our full pricing guide or contact us for a free fixed-fee quote.

Need a flood risk or drainage consultant? We provide planning-ready FRAs and SuDS drainage strategies across England and Wales.