How big can I build an extension without planning permission?

In England, a single-storey rear extension of up to 4m beyond the original rear wall on a detached house, or 3m on a semi or terrace, no more than 4m high — and up to 8m/6m via the prior-approval route. In Scotland the headline limits are 4m (3m on a terrace when building near a boundary), and in Wales up to 4m. All of this assumes an ordinary house outside a conservation area with its permitted development rights intact.

Do flats have permitted development rights?

Not the ones this article is about. The householder classes that let you extend — the 4m/3m rear extension rights and their Scottish and Welsh equivalents — apply to houses only, so a flat or maisonette needs planning permission to extend anywhere in the UK. That is narrower than "flats have no rights at all", which is how it usually gets repeated. Scotland in particular has a separate Class 4A covering specified external alterations to flats, with its own limits and with enlargement expressly excluded, and other classes pick up some ancillary works. If the job is an alteration rather than an extension, check the class rather than assuming.

Do I still need building regs if I don't need planning permission?

Yes, always. Planning permission and building regulations are separate regimes, and virtually every extension needs building regulations approval in England and Wales. In Scotland the equivalent is a building warrant — and it must be granted before work starts, not applied for retrospectively.

How much does a planning application for an extension cost?

The householder application fee is £548 in England (from April 2026), £371 in Scotland, and £585 in Wales (since December 2025). England's larger-home-extension prior-approval route is £249. Councils aim to decide householder applications in about eight weeks.

Can my neighbours block my extension?

Not by objecting alone. Councils can only weigh material planning considerations — privacy, overshadowing, overbearing impact, design. Loss of a view, effect on property value and construction nuisance aren't valid grounds. One objection rooted in planning policy carries more weight than a dozen unhappy neighbours.

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From the blog

Do you need planning permission for a house extension? The 2026 rules

13 August 2026 · Robert McLaggan

Most single-storey rear extensions in the UK don't need planning permission — they fall under permitted development. In England that means up to 4m beyond the rear wall for a detached house and 3m for a semi or terrace (or 8m/6m via the prior-approval route), no higher than 4m. Scotland and Wales run their own regimes with different numbers, so don't apply English rules north of the border. Permitted development is never no-paperwork: building regulations approval (England and Wales) or a building warrant obtained before work starts (Scotland) applies to virtually every extension, and a lawful development certificate is cheap insurance for when you sell. Where full permission is needed, the householder fee is £548 in England, £371 in Scotland and £585 in Wales, with a decision typically in about eight weeks.

The good news first: most single-storey rear extensions in the UK don't need planning permission at all. The less good news: the limits are different in England, Scotland and Wales, the exceptions catch people out, and "no planning permission" has never meant "no paperwork". Here's how it actually works in 2026.

The quick answer

Every ordinary house comes with permitted development (PD) rights — a national grant of permission for defined amounts of extension, no application needed. Stay inside the limits and you can build. Exceed them, live in a flat, or live somewhere designated, and you're applying to the council.

The limits are national — and the three nations genuinely differ, so start with the right column.

England

For a single-storey rear extension under permitted development:

Up to 4m beyond the original rear wall for a detached house; 3m for a semi or terrace
Maximum height 4m; within 2m of a boundary, eaves capped at 3m
Extensions and outbuildings together can't cover more than half the land around the original house

Want to go deeper? England has a second route: the larger home extension scheme allows single-storey rear extensions up to 8m (detached) or 6m (attached) with prior approval — a £249 application where the council notifies your neighbours (21 days to comment) and decides within 42 days. It's not available in conservation areas and the like.

Two-storey rear extensions can be PD too, but the rules tighten: maximum 3m projection, at least 7m to the rear boundary, matching roof pitch, obscure-glazed side windows upstairs. Side extensions are single-storey PD only, up to half the width of the original house.

One phrase does a lot of work here: the original house — as it stood in 1948 or when first built. A previous owner's extension already used up part of your allowance.

Scotland

Scotland runs its own regime, and the numbers are not England's:

Single-storey rear extensions: up to 4m from the rear wall (3m for a terrace) where the extension comes within 1m of a boundary; maximum height 4m, eaves 3m
The extension can't exceed the footprint of the original house, or cover more than half the rear garden
Extensions of more than one storey need a 10m setback from every boundary to be PD — which rules out most real gardens, so in practice two-storey extensions in Scotland need planning permission
No PD in conservation areas, and none of the extension rights for flats — though Scotland does have a separate class covering certain external alterations to a flat, short of enlarging it

Wales

Wales also has its own rules — close to England's, but not identical:

Single-storey rear extensions: up to 4m beyond the rear wall, maximum height 4m; within 2m of a boundary, eaves capped at 3m
Two-storey rear extensions: ground floor up to 4m, upper floor 3m, and the extended rear wall must be at least 10.5m from the rear boundary
The half-the-garden rule applies as in England
There is no 8m/6m prior-approval route in Wales — that scheme is England-only

What strips your rights

Whatever the nation, PD rights vanish or shrink if any of these apply:

You live in a flat or maisonette — none of the extension rights above, though in Scotland a separate class may still cover an external alteration that doesn't enlarge the flat
Designated land — conservation areas, National Parks, AONBs, World Heritage Sites
A listed building — listed building consent needed on top of everything else
An Article 4 direction — councils can remove PD rights street by street; five minutes on your council's website settles it
A condition on an old permission — some houses, especially on newer estates, had PD rights removed when they were built

If any of those ring true, ask the council before designing anything.

The paperwork you need even without planning

Two things survive "it's permitted development":

Building regulations (England and Wales) apply to virtually every extension — structure, insulation, drainage, electrics. In Scotland the equivalent is a building warrant, and the sequencing matters: it must be granted before work starts. Budget £600–£1,200 for building regs in England and Wales; Scottish warrant fees scale with the value of the works.
A lawful development certificate — optional, half the planning fee (£274 in England), and quietly one of the best-value documents in the process. It's the council formally confirming your extension was lawful, and it's what your buyer's solicitor will ask for when you sell. Without it, expect delays and indemnity-insurance haggling years from now.

If you do need permission

A householder application costs £548 in England, £371 in Scotland and £585 in Wales, plus a portal fee, and councils aim to decide in about eight weeks. Add your drawings — most people use an architect or architectural technician at this stage anyway.

And in England and Wales the Party Wall Act may apply separately — but it is triggered by what you are doing, not simply by how close you are. The three tests are building on the line of junction, work to a shared party structure, and excavating near a neighbouring building: within 3m and deeper than their foundations, or within 6m and deep enough to cut a line drawn at 45 degrees from the bottom of theirs. An extension near a fence with nothing next to it triggers none of them. Where one does apply, you serve formal notice in advance, and if the neighbour dissents you are into surveyors and a party wall award. Scotland has no Party Wall Act — boundary matters run under different rules entirely.

Two myths worth killing

"If it's up four years, it's legal." Not in England, not for anything recent. The enforcement window for unauthorised building work in England is now ten years — but the change is not retrospective, and the date is the one that matters: it applies where the work was substantially completed on or after 25 April 2024. Work finished before that keeps the old four-year window, so for an extension that went up in, say, 2021, four years is still the answer. Scotland and Wales run the four-year rule throughout — one more way the nations have quietly diverged.

"The neighbours will block it." Objections don't work the way people fear. Councils can only weigh material planning considerations — privacy, overshadowing, overbearing impact. Loss of view and property values are not planning matters, and one objection grounded in policy outweighs any number of angry ones. Talk to the neighbours early anyway; the extension will outlast the awkwardness either way.

Common questions

How big can I build an extension without planning permission?: In England, a single-storey rear extension of up to 4m beyond the original rear wall on a detached house, or 3m on a semi or terrace, no more than 4m high — and up to 8m/6m via the prior-approval route. In Scotland the headline limits are 4m (3m on a terrace when building near a boundary), and in Wales up to 4m. All of this assumes an ordinary house outside a conservation area with its permitted development rights intact.
Do flats have permitted development rights?: Not the ones this article is about. The householder classes that let you extend — the 4m/3m rear extension rights and their Scottish and Welsh equivalents — apply to houses only, so a flat or maisonette needs planning permission to extend anywhere in the UK. That is narrower than "flats have no rights at all", which is how it usually gets repeated. Scotland in particular has a separate Class 4A covering specified external alterations to flats, with its own limits and with enlargement expressly excluded, and other classes pick up some ancillary works. If the job is an alteration rather than an extension, check the class rather than assuming.
Do I still need building regs if I don't need planning permission?: Yes, always. Planning permission and building regulations are separate regimes, and virtually every extension needs building regulations approval in England and Wales. In Scotland the equivalent is a building warrant — and it must be granted before work starts, not applied for retrospectively.
How much does a planning application for an extension cost?: The householder application fee is £548 in England (from April 2026), £371 in Scotland, and £585 in Wales (since December 2025). England's larger-home-extension prior-approval route is £249. Councils aim to decide householder applications in about eight weeks.
Can my neighbours block my extension?: Not by objecting alone. Councils can only weigh material planning considerations — privacy, overshadowing, overbearing impact, design. Loss of a view, effect on property value and construction nuisance aren't valid grounds. One objection rooted in planning policy carries more weight than a dozen unhappy neighbours.

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