Do I need planning permission for a rear extension? The three routes, and what decides which one is yours

Do you need planning permission for a rear extension?

Often not, but the answer is a route, not a yes or no. There are three: permitted development with no application, prior approval for a deeper single-storey extension, and a full householder application. Which one you are on depends on the house, the land, the depth, the height and the number of storeys.

The Planning Portal puts the position plainly: many extensions can be built under permitted development, subject to limits and conditions, and rules vary where there is a planning condition, an Article 4 direction or another restriction. The Order is the source. Class A permits the enlargement, improvement or other alteration of a dwellinghouse, and paragraph A.1 lists what stops it applying. If any one of those limits is crossed, the whole extension needs permission, not just the extra part.

Planning permission is one of three separate approvals. Building regulations and the Party Wall etc. Act 1996 are different regimes, covered further down. Our guide to single storey extension planning permission goes through the single-storey limits letter by letter; this guide is the decision across all rear extensions.

How do earlier extensions use up your allowance?

The limits apply to the original house, defined in article 2(1) as the building as it existed on 1 July 1948, or as built if it was built after that date. They also apply to the total enlargement under A.1(ja): the new part together with any existing enlargement it will be joined to.

The 2019 MHCLG technical guidance gives worked examples. A detached house with an existing 4m single-storey extension cannot add 5m more under the 8m route, because the total would exceed 8m. A first-floor extension built above a single-storey extension that already goes beyond 3m is not permitted development either, because paragraph A.1(h) bars more than 3m on a multi-storey extension.

The 50% rule is separate. Under A.1(b), buildings in the curtilage other than the original house, including sheds, outbuildings and later extensions, may not cover more than 50% of the curtilage, excluding the ground area of the original house. A previous owner's extension counts, whether or not you know it was built. Old plans and the council's planning history search are the way to find out.

What does that spread mean for your rear extension?

It means a design that would pass in one borough can meet resistance in another. The statistics cannot say why: a low rate may reflect the schemes submitted, the housing stock or how the council reads neighbour amenity. Treat it as a reason to read your borough's guide and to consider pre-application advice, not as a verdict on any scheme.

The council's test is the same everywhere for the neighbour route: the impact on the amenity of adjoining premises. Sections through the boundary, window positions and accurate levels are what answer it.

Do you also need building regulations and a party wall notice?

Planning permission settles whether you may build. It does not settle how. The government's building regulations page says building regulations approval is different from planning permission, and you might need both. The Building Regulations 2010 cover the construction and extension of buildings.

The Party Wall etc. Act 1996 applies where you build on the line of junction, work on a party structure, or dig foundations within 3 metres (or, for deeper foundations, 6 metres) of a neighbour's building, as most rear extensions do. Under section 6, a notice of adjacent excavation is needed at least one month before digging starts. Under section 1, a notice is needed at least one month before building a new wall on the line of junction. Under section 3, a party structure notice must be served at least two months before the work, and no notice is needed where the adjoining owners and occupiers consent in writing. Our guide to planning permission and building regulations explains the split.

Should you get a lawful development certificate for a rear extension?

It is optional, and often worth it. Section 192 of the Town and Country Planning Act 1990 lets you apply to ascertain whether proposed operations would be lawful. If the council is satisfied, it must issue a certificate, and under section 192(4) lawfulness is conclusively presumed unless there is a material change before work begins. The government's guidance notes that there is no statutory requirement to consult neighbours.

For a rear extension the certificate is most useful when a buyer, lender or solicitor will later ask for proof that the work needed no permission. It says nothing about building regulations. Our guide to lawful development certificates explains the evidence to submit.

What happens if you build a rear extension without the right permission?

The council can take enforcement action, within time limits that depend on the date. Section 171B of the 1990 Act, as amended by the Levelling-up and Regeneration Act 2023 from 25 April 2024, gives councils ten years from substantial completion to take enforcement action against building operations in England. Under regulation 5 of SI 2024/452 the change does not apply to operations substantially completed before 25 April 2024, so in broad terms the previous four-year period still applies to them. Whether and when work was substantially complete is a question of fact, and different rules can apply where a breach was concealed, so take advice before relying on any date.

Do not treat the time limit as a plan. A buyer's solicitor will ask for proof, and article 3(5) of the Order says permitted development rights do not apply where the building operations involved in constructing the existing building are unlawful. If you inherited an unpermitted extension, take advice on its status before relying on any date.

What is the simplest way to settle rear extension planning permission?

Check the house, the land and the original rear wall, in that order. Those three checks settle the route before any drawing is made. If the answer is not clear, send us the address and a photograph of the rear. We check the constraints for free and tell you whether you need a certificate, prior approval or an application. Our rear extension drawings service covers all three, and extension plans in London covers larger schemes.