Often not — but "often" is doing a lot of work in that sentence. Plenty of single-storey rear extensions in England can be built under permitted development rights, with no planning application at all. But the limits are strict, they're measured in specific ways, and a surprising number of houses are quietly excluded from them altogether.
This guide covers the rules for houses in England as they stand at the time of writing. (Flats and maisonettes don't have these rights, and Scotland, Wales and Northern Ireland have their own rules.)
The short version
A single-storey rear extension can usually go ahead without planning permission if all of these are true:
- It extends no more than 3 metres beyond the original rear wall of an attached or semi-detached house, or 4 metres for a detached house
- It's no more than 4 metres high
- If any part is within 2 metres of a boundary, the eaves are no higher than 3 metres
- It's no higher than the highest part of the existing roof, and the eaves are no higher than the existing eaves
- The extension and any other buildings cover no more than half the land around the original house
- It uses materials of a similar appearance to the existing house
- It doesn't include a veranda, balcony or raised platform
Miss any one of those and you're into planning permission territory.
Going bigger: the prior approval route
For single-storey rear extensions, there's a middle route. On most sites you can build up to 6 metres (attached or semi-detached houses) or 8 metres (detached houses) under the larger home extension scheme — but you have to apply for prior approval first.
Prior approval isn't a full planning application. The council notifies your neighbours, and if any of them object, the council considers the effect on their amenity — things like light and outlook — before deciding. It's a lighter process than a full application, but it isn't automatic, and it isn't available everywhere (see conservation areas below).
What about two-storey rear extensions?
Two-storey rear extensions are much more tightly restricted. Under permitted development they can't extend more than 3 metres beyond the rear wall, can't come within 7 metres of the rear boundary, and the roof pitch has to match the existing house as far as practicable. In practice, many two-storey extensions end up needing a planning application.
When you'll need planning permission regardless
Even a modest extension needs planning permission if:
- It's in a conservation area or other "designated land" — all rear extensions of more than one storey need permission there, and the 6/8 metre prior approval route isn't available. (More on this in our guide to conservation areas.)
- Your house is a flat or maisonette, or was created through a change of use (for example, a converted office or barn). These don't have the same permitted development rights.
- An Article 4 direction applies. Councils can remove permitted development rights in specific areas, and they often do in conservation areas and around listed buildings.
- A planning condition removed your rights. This is common on newer housing estates — the original planning permission for your house may have taken away the right to extend without applying.
- The house is listed. Listed building consent is a separate requirement and applies to almost any alteration.
- You've already extended. Permitted development limits are measured against the original house — as it was first built, or as it stood on 1 July 1948. A previous extension (yours or a past owner's) counts against your allowance.
That last point catches people out more than any other. If your house has already been extended, you may have far less room under the limits than you'd assume.
Permission isn't the only approval
Even if your extension is fully permitted development, you'll almost certainly still need building regulations approval — a separate system covering how the extension is built (see Planning Permission vs Building Regulations). And if you're building on or near a shared boundary, the Party Wall etc. Act may apply too.
How to be certain
Don't rely on a rule of thumb, and don't rely on a neighbour who "didn't need permission for theirs" — their house may have different rights. There are two reliable ways to get a definite answer:
- Get the plans checked against the limits before you commit to a design. An hour of someone measuring the existing house and testing your proposal against the rules is cheap compared with enforcement action later.
- Apply for a Lawful Development Certificate. This is a formal confirmation from the council that your proposed extension is lawful as permitted development. It costs half the householder planning fee, and it's worth having — it protects you when you come to sell, when a buyer's solicitor will ask for evidence.
Not sure which side of the line you're on?
Tell us about your house and what you'd like to build, and we'll tell you honestly whether it needs an application — before you spend anything on drawings. Get in touch, or use our online estimator to see what an extension might cost, including any planning fees.