It’s the first question most people ask us, and the honest answer is: it depends on the size of the extension, where it goes and where your house is. The good news is that many single-storey extensions don’t need a full planning application at all. Here’s how it works.
Permitted development: extending without a planning application
Most houses in England come with “permitted development” rights. These let you extend within set limits without applying for planning permission. For a single-storey rear extension, the main limits are:
- It can go out up to 4 metres from the back of the original house if it’s detached, or 3 metres for a semi or terrace.
- It can be no more than 4 metres high, and no more than 3 metres at the eaves if it’s within 2 metres of a boundary.
- Extensions and outbuildings together can’t cover more than half the land around the original house.
- Materials should look similar to the existing house.
Side extensions and two-storey extensions have their own, tighter rules. “Original house” means the house as it was first built (or as it stood in 1948), so earlier extensions count towards your allowance.
Going bigger: prior approval
A larger single-storey rear extension, up to 8 metres for a detached house or 6 metres for others, can often go ahead through the “prior approval” route. You tell the council what you’re planning, your neighbours are notified, and the council has a set time to decide whether it affects them too much. It’s simpler than a full application, but it isn’t automatic.
When you’ll need full planning permission
- Your extension goes beyond the permitted development limits.
- Your house is a flat or maisonette (these don’t have permitted development rights).
- You’re in a conservation area or a National Park. The New Forest is a National Park, so homes there have tighter rules.
- The council has removed permitted development rights for your street or your house (an “Article 4 direction” or a condition from when the house was built).
- The house is listed, in which case you’ll also need listed building consent.
Building Regulations are a separate thing
Even if you don’t need planning permission, almost every extension needs Building Regulations approval. Planning is about how the extension looks and how it affects your neighbours. Building Regulations are about how it’s built: foundations, structure, insulation, drainage, fire safety and ventilation. A building control inspector checks the work at key stages and issues a completion certificate at the end. Keep it safe, because you’ll need it when you sell.
Our advice
- Check before you commit. The government’s Planning Portal has an interactive guide, and your council’s planning team (Southampton City Council, Test Valley, Eastleigh, Winchester or the New Forest National Park Authority, depending on where you live) can confirm what applies to you.
- Consider a Lawful Development Certificate. If you’re relying on permitted development, this certificate from the council confirms your extension is lawful. It’s not compulsory, but it’s reassuring and useful when you sell.
- Think about your neighbours early. If you’re building on or near a shared boundary, the Party Wall Act may mean you need to give them notice.
Rules change from time to time, so always check the current position for your property. If you’d like us to take a look, we’ll come and see your home, talk through what’s likely to be allowed and give you a clear written quote. Call us on 07788 975194.