It’s one of the most-searched questions in UK construction, and the answer is: often not — but it depends. Many house extensions in England can be built under “permitted development” without a full planning application, while others need permission or a prior-approval notification. Here’s how to tell which camp your project is in. (This covers England; Scotland, Wales and Northern Ireland have their own rules.)

The short answer

Permitted development rights let homeowners carry out certain extensions and alterations without applying for planning permission, provided they stay within set limits. Go beyond those limits — or live somewhere the rights are restricted — and you’ll need a planning application. The honest move on any non-trivial job is to confirm with the local planning authority before you build, ideally via a Lawful Development Certificate, which is formal proof the work is lawful.

What permitted development covers

For a typical house, permitted development can include rear extensions, side extensions, loft conversions, outbuildings and more — each with its own size and height rules. The key conditions that run through all of them:

The extension mustn’t cover more than half the area of land around the original house.
It mustn’t be higher than the highest part of the existing roof, or forward of the principal elevation facing a road.
Materials should be similar in appearance to the existing house.
On designated land (see below) the rights are tighter.

Rear extensions: 3m/4m vs 6m/8m

This is the rule people search for most. For a single-storey rear extension under standard permitted development:

Up to 3m deep for a terraced or semi-detached house.
Up to 4m deep for a detached house.
Maximum height of 4m.

You can go deeper — up to 6m (terraced/semi) or 8m (detached) — under the Larger Home Extension scheme, but this isn’t automatic: it requires a prior approval application with a neighbour consultation, where the council notifies adjoining neighbours and can block it if they object on impact grounds. So a 6m/8m extension still means a form and a wait, just not a full planning application.

Side and two-storey extensions

Side extensions under PD must be single-storey, no higher than 4m, and no wider than half the width of the original house. Two-storey rear extensions can be permitted development on detached and some semi-detached homes within strict limits (for example not extending beyond 3m and staying a set distance from the rear boundary), but they trip people up — many two-storey jobs do need full permission. Check carefully before committing.

When PD rights don’t apply

Permitted development is reduced or removed entirely in a number of common situations:

Listed buildings — almost any alteration needs Listed Building Consent.
Conservation areas, National Parks, AONBs — tighter limits on what’s permitted.
Article 4 Directions — councils can withdraw PD rights for specific areas or even specific alterations.
Flats and maisonettes — PD rights for extensions generally don’t apply; you’ll need permission.
Houses already extended — earlier extensions count against your allowance.

Because designations and Article 4 directions vary street by street, two identical extensions on neighbouring roads can have completely different requirements. Always check the specific property.

Don’t forget Building Regulations

A crucial distinction: planning permission and Building Regulations are two separate things. Even when an extension doesn’t need planning permission, it almost always still needs Building Regulations approval — structure, insulation, fire safety, drainage and more. “Permitted development” means you can build it without a planning application; it does not mean you can skip Building Control.

The honest summary

Many single-storey extensions in England fall under permitted development (3m/4m deep, or 6m/8m with prior approval), but listed buildings, conservation areas, flats, Article 4 areas and previous extensions can all change the answer — and Building Regulations apply either way. When in doubt, a Lawful Development Certificate is cheap insurance. For contractors: a validated planning application or prior-approval notice is your signal that a homeowner has committed to the work and needs a builder. House of Planning Service tracks them all the day they’re validated.

Do I need planning permission for an extension? (2026 UK guide)