Ask three homeowners how far they can extend without planning permission and you'll get three different numbers. The owner of the semi will confidently quote the detached allowance. Someone will insist the "45-degree rule" caps everything. And at least one will believe the whole thing needs council sign-off anyway, so why bother learning the limits.
The rules are actually precise. Permitted development is a set of exact, measurable numbers written into law, and if your extension fits inside them, you can build without a planning application. Get one number wrong, though, and you've built an unauthorised development that the council can order you to tear down. So the numbers are worth ten minutes of your attention.
Everything below applies to England. Wales, Scotland and Northern Ireland each run their own permitted development regimes with genuinely different figures (Scotland allows 4m for every house type but caps garden coverage at 30%, for instance). If your house is outside England, check your national rules before trusting anything here.
What Permitted Development Actually Is
Permitted development (PD) isn't an exemption from planning permission. It IS planning permission, granted in advance by the government for a defined class of building work, through the Town and Country Planning (General Permitted Development) (England) Order 2015. Parliament has already said yes to every extension that meets the conditions. Your council has no say, no discretion, and no application to process.
That's why the limits are so specific. A blanket national permission can't weigh up each site individually, so it draws hard numeric lines instead. Meet every one and you're permitted. Miss any one and you're not, even by a centimetre on paper.
The Rear Extension Numbers
For a single-storey rear extension, the envelope looks like this:
| Limit | Semi-detached or terraced | Detached |
|---|---|---|
| Depth beyond the original rear wall (standard) | 3m | 4m |
| Depth with prior approval (Larger Home Extension) | Up to 6m | Up to 8m |
| Maximum overall height | 4m | 4m |
| Maximum eaves height within 2m of a boundary | 3m | 3m |
The single most common mistake with this table: assuming the detached figure applies to you. It doesn't unless your house is fully detached. Semi-detached and terraced houses get 3m, and forum threads are full of people who designed to 4m, briefed a builder, and only discovered mid-quote that they were a metre over the line for their house type.
Two measurement points matter. Depth is taken from the original rear wall, meaning the house as first built (or as it stood on 1 July 1948 for older properties), and it's measured from the external wall face rather than the guttering. Don't design to the last centimetre regardless: planners and building control have been known to measure differently at the margins, and an extension that's compliant with 100mm to spare never has that argument.
The boundary rule deserves its own sentence because it quietly reshapes a lot of designs. If any part of the extension sits within 2m of a boundary, the eaves on that extension can't exceed 3m. On a typical terrace or semi where the extension runs close to the fence, that 3m eaves cap, not the 4m height cap, is what actually constrains your roof design.
The full condition set, including the awkward cases (wrap-arounds, outriggers, L-shaped originals), is in the tree's permitted development rules leaf. And a note on stability: a 2024 government consultation floated loosening these limits, but nothing has been enacted as of mid-2026. The figures above are the law in force.
The 6-Metre and 8-Metre Rule: Prior Approval
Depth beyond 3m (or 4m detached) doesn't automatically mean a planning application. Sitting between standard PD and full planning is the Larger Home Extension scheme: single-storey rear extensions up to 6m for attached houses and 8m for detached ones, subject to a process called prior approval. The scheme was introduced as a temporary measure in 2013, renewed twice, and made permanent in 2019, so if you've seen both dates quoted online, both are right.
Prior approval is not a planning application and it's not a free pass either. You tell the council what you intend to build before starting. The council writes to the adjoining neighbours, who get 21 days to make representations. If nobody objects, the council has 42 days in total to respond, and silence past the deadline counts as deemed approval. If a neighbour does object, the council assesses the impact on that neighbour's amenity and can refuse.
Be clear-eyed about that last part. Refusals happen. A 6m extension that meets every numeric limit can still be turned down after a neighbour's loss-of-light objection, because meeting the numbers gets you into the process, not through it. The consolation is that the refusal test is narrow (impact on neighbouring amenity only), far narrower than the "material considerations" a full planning application is judged against.
The fee is £249, plus a service charge of around £91 if you submit through the Planning Portal; both figures change periodically, so check the Planning Portal's fee calculator before budgeting. The mechanics, the drawings you need, and how to handle an objection are covered in the prior approval for larger home extensions leaf.
Warning
The 45-Degree Rule Is Not a Permitted Development Rule
Search for extension rules and you'll run into "the 45-degree rule" presented as if it were one thing. It's three things, and mixing them up leads people to shrink designs that were fine or approve designs in their head that weren't. Here's the disambiguation nobody else seems to publish.
The daylight rule (the one most people mean). Planning officers assessing a full planning application often draw a line at 45 degrees from the centre of the nearest habitable-room window of the neighbouring house, in plan and in section. If your extension crosses that line, they may object on loss of light or outlook. This comes from BRE guidance on daylight and sunlight, not from legislation. It's a judgement tool, applied with varying strictness from one council to the next (some publish their own 25- or 30-degree variants), and it has no role in deciding whether your scheme qualifies as permitted development. A PD-compliant extension is judged against the numeric limits above and nothing else. The one partial exception: on the 6m/8m prior-approval route, a neighbour's objection about light is assessed as amenity impact, though even there the officer isn't bound to the 45-degree geometry.
The highway test (this one does affect PD). Government technical guidance on permitted development uses its own 45-degree test to decide whether a side wall counts as "fronting a highway". A side wall angled at 45 degrees or less from the road is treated as fronting it, and walls fronting a highway lose their PD extension rights (more on that below). Different rule, different purpose, same two words.
The Party Wall Act trigger (nothing to do with planning at all). If you excavate within 6m of a neighbouring building, and a line drawn downwards at 45 degrees from the bottom of their foundations would cut into your excavation, you must serve notice under the Party Wall etc. Act 1996. This governs how you notify neighbours about foundation digging. It applies whether your extension is PD, prior approval, or full planning, and it has zero bearing on which of those routes you need.
If someone tells you "you can't do that, the 45-degree rule", your first question is: which one? Two of the three probably don't apply to your situation, and the one that does may not say what they think it says.
Side Extensions and Two-Storey Rear Extensions
Rear extensions get the generous treatment. Everything else is tighter.
A side extension can be permitted development, but only if it's single storey, no more than 4m high, and no wider than half the width of the original house. All three at once. A two-storey side extension is never PD, whatever its size. And width disputes at the margin are real: whether a bay window counts towards the "width of the original house" has decided actual refusals, so measure the plain wall-to-wall width and treat anything marginal as a question for the council, not a judgement call.
A two-storey rear extension has its own pair of limits: no more than 3m beyond the original rear wall, and no part of it within 7m of the boundary opposite the rear wall. That second condition kills the idea for most short gardens before the first one is even measured.
Half Your Garden: The 50% Rule
Underneath all the depth and height limits sits a cumulative cap. The total footprint of everything built in the garden, meaning extensions, sheds, garages, outbuildings, a previous owner's conservatory, all of it, must not exceed 50% of the land around the original house.
The phrase doing the heavy lifting there is original house. The baseline is the house as first built, or as it stood on 1 July 1948. Extensions added since then, by you or by anyone who owned the house before you, count against the allowance. You cannot reset the clock by buying an already-extended house. Plenty of buyers of extended Victorian terraces discover their PD allowance was spent decades before they got the keys, sometimes entirely.
So before you design anything, work out two things: where the original rear wall was, and what's already been built within the plot. Old planning records on the council's website, historic maps, and the mortgage survey all help. If the house has been extended before and nobody can show what authorised it, that's a second problem worth solving at the same time.
Materials, and Why the Front of the House Is Off Limits
Two conditions catch people who've done all the measuring correctly.
Materials used on the exterior must be "similar in appearance" to the existing house. Similar, not identical, and that vagueness cuts both ways: councils interpret it inconsistently, with render-over-brick and timber cladding the recurring flashpoints. If your design deliberately contrasts with the original house, glass box against Victorian brick, say, that's a design decision for a full planning application, not a PD scheme. On designated land the position is stricter again: exterior cladding of any kind is excluded from PD entirely.
And nothing extends forward under PD. No extension is permitted beyond a wall that forms the principal elevation of the house, or beyond a side wall that fronts a highway (the 45-degree highway test above decides which walls those are). In practice: front extensions need planning permission, and on corner plots the side facing the road usually does too.
What Voids Permitted Development Entirely
Everything above assumes your house still has its PD rights. Several situations remove or cut them before you've measured anything:
- Flats and maisonettes. Householder PD rights belong to houses. A flat has none, including ground-floor flats with their own gardens.
- Listed buildings. Listed building consent sits on top of everything, and assuming any work is permitted without checking is how criminal liability happens. Yes, criminal: unauthorised works to a listed building are an offence, not just an enforcement matter.
- Designated land. Conservation areas, National Parks, Areas of Outstanding Natural Beauty, the Broads and World Heritage Sites. Standard single-storey rear PD survives here, but side extensions, multi-storey rear extensions, exterior cladding and the entire 6m/8m prior-approval scheme do not.
- Article 4 directions. A council can strip specific PD rights from a defined area, most commonly inside conservation areas. Several London boroughs have done exactly this for extensions and roof alterations.
- Conditions on old planning permissions. A previous permission for the property can carry a condition removing PD rights from that house permanently. It binds every future owner and shows up nowhere except the decision notice itself.
That last one is the quiet menace. Search your address on the council's online planning register and read the conditions on every historic permission before you spend a pound on design work. Ten minutes, and it rules out the nastiest surprise in this list.
The Lawful Development Certificate: Buy the Proof
Suppose your extension passes every test above. You could just build it. Nothing in law requires you to tell the planning department anything first.
Don't. Apply for a Lawful Development Certificate (LDC) before you build. It's the council's formal, binding confirmation that your scheme is lawful, it costs £274 in England (fees are reviewed annually, so check the Planning Portal), and unlike planning permission it never expires. If the facts fit the rules the council must issue it; there's no discretion and neighbours have no say.
What you're really buying is the answer to a question that will be asked years from now, by someone with money on the line. When you sell, the buyer's solicitor will ask what authorised the extension. "We measured it and it was fine" makes conveyancers reach for indemnity policies and makes some lenders hesitate. A certificate closes the question in one document. It also freezes your position: if the council later brings in an Article 4 direction, a pre-existing LDC protects the scheme it describes.
Tip
If the self-assessment feels marginal on any condition, £100-300 for a planning consultant's compliance check is cheap insurance against certifying your own mistake.
When Full Planning Is the Better Call Anyway
Permitted development is a route, not a destination. Sometimes the right decision is a planning application even though a smaller PD scheme was available.
A real 30m² rear kitchen extension in Oxfordshire made exactly that call. The design brief wanted depth at the edge of the PD envelope plus roofline changes, so rather than self-certify against a stack of conditions, the build went through a full householder application: submitted in spring 2021, granted May 2021, roughly eight weeks start to finish. The application banked a council-approved drawing set before any digging started, which is the same certainty an LDC provides for a PD scheme, bought a different way. The project it protected wasn't small: construction came to £120,480 excluding the kitchen itself (about £2,190/m²), and £172,900 all-in (about £3,143/m²). Against numbers like those, the cost of getting the permission route wrong isn't the application fee. It's the risk to everything built on top of it.
The general lesson: if your design wants more than the PD numbers allow, or the conditions leave you genuinely unsure, stop trying to squeeze into the envelope and apply properly. The do you need planning permission leaf walks the whole decision, with the checks in order.
What to Do Next
Work the sequence, in this order:
- Check for disqualifiers. Flat, listed building, designated land, Article 4 direction, or a PD-stripping condition on an old permission. Any of these and you're on the planning application route regardless of size.
- Establish the original house. Find the original rear wall and add up everything already built in the garden against the 50% cap.
- Measure your design against the limits. Depth, height, eaves near the boundary, side-extension width if relevant.
- Pick the route. Inside the standard limits: apply for an LDC, then build. Between 3-6m (attached) or 4-8m (detached): prior approval, and wait for the outcome. Outside all of it, or marginal: full planning application.
Permission is the first gate of the project, not the project. The kitchen extension tree maps every task from this decision through foundations, structure and fit-out to the completion certificate, in build order, and the structure is free to browse: see the full tree. The Access Pass (£49) opens the working detail on every task, including the PD self-assessment checks, the prior-approval walkthrough, and the LDC application steps for this phase. The first post in this series covers the wider question of whether a kitchen extension needs planning permission at all.
Frequently Asked Questions
What is the 6-metre and 8-metre rule for extensions?
It's the Larger Home Extension scheme. A single-storey rear extension can go beyond the standard permitted development depth, up to 6m from the original rear wall for semi-detached and terraced houses or 8m for detached houses, if the council grants prior approval first. Neighbours get 21 days to comment and the council has 42 days to decide. The scheme started in 2013 as a temporary measure and became permanent in 2019. It isn't available on designated land such as conservation areas.
Is prior approval the same as planning permission?
No. Prior approval is a notification-and-consent step inside permitted development. The council checks a much narrower question than a planning application: whether the extension would harm the neighbours' amenity, and only if a neighbour objects during the 21-day consultation. It's faster and cheaper than full planning, but it isn't automatic, and starting work before the process finishes leaves you building without permission.
Do side extensions count as permitted development?
They can, under stricter limits than rear extensions: single storey only, maximum 4m high, and no wider than half the width of the original house. Two-storey side extensions are never permitted development. On designated land (conservation areas, National Parks, AONBs), side extensions lose PD rights entirely and need a planning application whatever their size.
What voids permitted development rights?
Living in a flat or maisonette (no householder PD rights at all), a listed building, designated land such as a conservation area or AONB (which removes side extensions, multi-storey rear extensions and the 6m/8m scheme), an Article 4 direction covering your area, or a condition on a previous planning permission that stripped PD rights from your specific property. The last two are invisible until you search the council's planning register, so search it before designing anything.
Do I need a Lawful Development Certificate if my extension is permitted development?
You're not legally required to have one, but get one anyway. The certificate is the council's binding confirmation that the extension is lawful, it costs £274 in England, and it never expires. Without it, when you sell, you're asking a buyer's solicitor and mortgage lender to take your own tape measurements on trust. Apply before you build so the certificate is assessed against drawings rather than against the finished structure.
Is the 45-degree rule a permitted development rule?
The one people usually mean, the daylight sightline drawn from a neighbour's window, is not. It's a non-statutory guideline planning officers use when assessing full planning applications, and it plays no part in whether a scheme qualifies as permitted development. Two other 45-degree tests do exist: a highway-fronting test in the PD technical guidance that decides whether a side wall loses its PD rights, and the Party Wall Act's 45-degree excavation-notice trigger, which is about notifying neighbours of foundation digging, not planning at all.