KNOW THE RULES

Do I need planning permission for a single-storey rear extension?

Often yes under permitted development, but exceptions and size limits mean you should check before building.

KR-01The short answerIt depends. Many single-storey rear extensions in England can be built under 'permitted development' without planning permission, as long as they stay inside set size and height limits. If your extension goes over those limits, or your property is an exception (flat, listed building, conservation area, Article 4 land), you will need full planning permission.
KR-02Check which part of the UK you're inThis guide covers England first, because planning rules differ across the UK. Wales has its own permitted development rules, and Scotland and Northern Ireland run separate planning systems entirely. See the step near the end for Wales, Scotland and Northern Ireland differences.
KR-03Check the size and height limitsIn England, permitted development usually allows a single-storey rear extension up to 4 m beyond the rear wall for a detached house, or 3 m for a semi-detached or terraced house, without needing neighbour notification. Larger extensions (up to 8 m detached, 6 m other houses) may be allowed under the 'larger home extension' neighbour consultation scheme. Height limits also apply, including a lower eaves height near boundaries. These figures can change, so confirm the current limits on the Planning Portal before assuming they apply to you.
RULES
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Permitted development rights are a legal exemption from planning permission, and you must stay within the current size and height limits to rely on them.

KR-04Check the exceptionsPermitted development rights do not apply to flats or maisonettes, so extensions there almost always need planning permission. If your house is listed, an extension will almost certainly need listed building consent as well as planning permission, whatever its size. Conservation areas and other designated land (National Parks, AONBs) have tighter permitted development limits, and an Article 4 direction can remove permitted development rights entirely for your street. If your house has already been extended before, you may have used up your permitted development allowance, and leasehold flats or houses with restrictive covenants may ban extensions even where planning law allows them.
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Listed building consent is a separate legal requirement from planning permission, and works without it are a criminal offence.

KR-05Planning permission and building regulations are separateNot needing planning permission does not mean you can skip building regulations. Building regulations cover structural safety, insulation, fire safety and drainage, and they apply to almost all extensions regardless of planning status. You need to satisfy both planning rules and building regulations, and they are checked by different people.
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Building regulations approval is a separate legal requirement from planning permission, and most extension work must comply with it.

KR-06Who to ask and howUse the Planning Portal's interactive guidance to check if your extension counts as permitted development, and consider applying for a Lawful Development Certificate as written proof if you are not applying for full planning permission. For building regulations, contact your council's Building Control team or a registered building control approver before work starts. If the extension sits on or near a shared boundary wall, you may need a party wall surveyor under the Party Wall Act.
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The Party Wall Act requires you to serve notice on neighbours before certain work on or near a shared boundary.

KR-07Realistic costs and timescalesIn England, a householder planning application costs £548 and a Lawful Development Certificate (written proof you don't need permission) costs £274, at the rates in force from April 2026. The 'larger home extension' prior approval costs £249. Councils normally decide within 8 weeks once the application is valid. Building Regulations fees are separate and set by your council or building control approver. Fees rise most years, so check the Planning Portal before you apply.
KR-08What happens if you skip itBuilding without required planning permission or building regulations approval can lead to enforcement action, including being told to alter or demolish the extension. Unauthorised work can also hold up or kill a house sale, because buyers' solicitors ask for proof of compliance. Indemnity insurance can sometimes cover a missing certificate, but it is a last resort and does not fix an unsafe or unlawful structure.
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Councils have legal powers to take enforcement action against unauthorised building work.

KR-09If you live in Wales, Scotland or Northern IrelandWales has its own permitted development rules, which are similar to England's but not identical, so check with your local council or the Planning Portal's Wales guidance rather than assuming English limits apply. Scotland uses a different system of building warrants and its own permitted development rules; check mygov.scot before you plan anything. Northern Ireland runs a separate planning system entirely, with its own rules on permitted development and building control, covered on nidirect.
RULES
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Scotland and Northern Ireland have their own planning and building control laws, separate from England and Wales.

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