Living in Flood Zone 2 or 3 doesn’t stop you extending your home. Councils approve extensions in flood risk areas every day. But there are a few extra rules, and some smart design choices that make the extension – and the rest of the house – cope better with flooding.
Do you need planning permission?
Many single-storey rear extensions can be built under permitted development rights without a planning application, subject to size limits and conditions. If your project needs planning permission, the application will need a flood risk assessment because the home is in Zone 2 or 3.
Extensions to existing homes don’t face the ‘sequential test’ that applies to new homes, so you won’t be asked to show there’s a lower-risk site elsewhere.
Floor levels
The Environment Agency’s standing advice for minor extensions says floor levels should be either no lower than the existing floor levels, or 300mm above the estimated flood level. Where raising the floor isn’t practical, you should build in flood resilience and explain this in your assessment.
Avoid creating new ground-floor or basement bedrooms in high-risk areas – sleeping accommodation below the likely flood level is the most dangerous place to be.
Building in resilience
Resilient construction means that if water does get in, it does less damage and the home can be back in use quickly. For an extension, consider:
- solid concrete floors with tiles rather than timber floors and carpets;
- lime or cement-based plaster, or water-resistant boards, instead of gypsum plaster on lower walls;
- electrical sockets, boilers and meters raised well above floor level;
- kitchen units made from plastic or stainless steel, or freestanding furniture;
- a non-return valve on drains, so water can’t back up into the house.
See property flood resilience for what’s worth the money.
Don’t make flooding worse
Your extension shouldn’t push water onto neighbours. Planners look at how rainwater from the new roof and paving will be handled – soakaways, permeable paving or water butts can help. In Zone 3, an extension can also take up space that floodwater would otherwise fill, so larger extensions may need to show how that’s compensated for.
Insurance
Tell your insurer about building work before it starts – some policies change cover during construction – and update the rebuild value once it’s finished. Keep records of any resilience measures, as they can help at renewal.
Talk to your council’s planning team early, and check whether the Environment Agency holds flood level data for your site.
Last reviewed November 2026. This guide is general information about England, not legal, financial or insurance advice. Check the Environment Agency’s own services and speak to a professional before making decisions about a specific property.
