Type “how big can I extend without planning permission” into a search engine and the honest answer is: it depends on more than the size of the extension. Position matters. Height matters. What previous owners already built matters. Whether the house is detached, semi-detached or terraced matters. A single figure — “three metres” or “four metres” — is only ever part of the picture, and relying on it alone is exactly how homeowners end up with an extension that isn’t actually permitted development, sometimes discovering this only when they come to sell.
This article covers what determines whether an extension qualifies, in England, under Part 1 Class A of the General Permitted Development Order (GPDO). It doesn’t cover whether the extension is a good idea, what it should look like, or how to build it — that’s the territory of our separate Extensions & Conversions coverage. This is specifically about the planning threshold: can it rely on permitted development rights, or does it need a full planning application?
The government’s householder permitted development technical guidance is the detailed reference behind these rules.
The basic single-storey rear extension limits
For a straightforward single-storey rear extension, measured from the rear wall of the original house — not necessarily the rear wall you see today — the standard depth limits are:
| House type | Standard limit | Larger extension route |
|---|---|---|
| Detached | 4 metres | 8 metres |
| Semi-detached or terraced | 3 metres | 6 metres |
Height matters alongside depth. The overall height of a single-storey rear extension is generally capped at 4 metres. Where any part of the extension comes within 2 metres of a boundary of the property’s curtilage, the eaves height is capped at 3 metres — a rule that catches out a lot of extensions built close to a side boundary with an ambitious roof design.
Those figures are not a complete permission in themselves. The proposal still has to satisfy the other relevant Class A limits and conditions, including rules on position, existing extensions, materials, curtilage coverage and any restrictions affecting the particular property.
The larger extension route isn’t a simple upgrade
The right-hand column above — up to 8 metres for a detached house and 6 metres for any other house — isn’t automatically available.
It’s accessed through a specific prior approval process commonly known as the larger home extension or neighbour consultation scheme. The homeowner must apply to the council before work starts, and adjoining owners or occupiers are notified. If qualifying objections are made, the council considers the effect of the proposed extension on the amenity of adjoining properties.
It is genuinely still permitted development, in the sense that a conventional householder planning application isn’t required, but it isn’t a matter of simply building bigger and hoping nobody minds. We’ve given this process its own detailed article, since its notification, consultation and timing requirements are easy to misunderstand.
The larger extension route is single-storey only. It is also unavailable on Article 2(3) land — which includes conservation areas, National Parks, the Broads, World Heritage Sites and Areas of Outstanding Natural Beauty, now generally known as National Landscapes — and on Sites of Special Scientific Interest. Other, more limited permitted development rights may still apply depending on the site and proposal.
The Planning Portal extension guidance provides a useful overview of the different limits.
Extensions of more than one storey
Extensions of more than one storey sit under a stricter version of the Class A rules.
The permitted depth is limited to 3 metres beyond the rear wall of the original house, regardless of house type, and the enlarged part must not come within 7 metres of any boundary of the curtilage opposite the rear wall of the house.
The roof pitch should match the existing house as far as practicable, and upper-floor windows in a side-facing wall or roof slope must be obscure-glazed and non-opening unless the opening part is more than 1.7 metres above the floor.
Other Class A restrictions still apply, including the overall roof and eaves height rules. And on Article 2(3) land, a rear extension resulting in an enlargement of more than one storey is not permitted development under Class A.
This is a noticeably tighter set of conditions than the single-storey rules, and it’s a common point of confusion for homeowners assuming their single-storey allowance simply doubles for an upper floor.
Side extensions
A side extension under Class A must be single-storey, no more than 4 metres high and no wider than half the width of the original house, measured at its widest point.
Where the proposed extension joins an existing enlargement, the combined or “total enlargement” can matter when applying these limits.
There is also an important position rule: an extension cannot project beyond a wall forming the principal elevation of the original house, or beyond a side elevation where that wall fronts a highway, and remain permitted development under Class A.
On Article 2(3) land, extensions beyond a side wall of the original house are not permitted development under this class.
Original house: why an old extension can shrink what you can still build
This is one of the most consequential rules in the whole system, and one of the least understood.
The rear-extension limits are measured against the original house — the property as it was first built, or as it stood on 1 July 1948 if older than that — not simply against the house as it currently stands.
Existing extensions can therefore affect what a new proposal can do, particularly where the proposed extension is joined to an earlier enlargement. The Class A rules require the total enlargement — the proposed work together with the existing enlargement it joins — to remain within the relevant limits.
Worked example: suppose a semi-detached house was built in 1970 and already has a 3-metre single-storey rear extension added by a previous owner. The current owner wants to add another 2 metres directly onto the back of it.
That isn’t treated as a fresh 2-metre extension with a new 3-metre allowance. The joined enlargement would extend a total of 5 metres beyond the rear wall of the original 1970 house.
It would therefore exceed the standard 3-metre rear-extension limit. Depending on the property and the rest of the proposal, it might potentially fall within the larger home extension route — up to 6 metres for a semi-detached house — but only if all the relevant conditions are satisfied and the prior approval process is completed.
The ownership history doesn’t reset anything. The relevant physical reference point remains the original house.
What if the old extension is demolished first? This is where the position is more nuanced than the common idea that a previous extension permanently “uses up” an allowance.
An extension that has genuinely been demolished is no longer an existing enlargement occupying the site. A replacement proposal is still measured from the rear wall of the original house and has to satisfy the current Class A rules, but the demolished building does not continue indefinitely as a fictional existing extension for the calculation.
That doesn’t mean demolition automatically makes the new scheme permitted development — planning conditions, Article 4 Directions, the property’s history and the design of the replacement may still affect the answer — but there is no general rule that a vanished extension permanently consumes the depth allowance forever.
The 50% curtilage rule
Separately from the depth and height limits on any particular extension, Class A places a cumulative cap on how much ground within the curtilage can be covered by buildings.
As a result of the proposed work, the total area of ground covered by buildings within the curtilage — excluding the original dwellinghouse itself — must not exceed 50% of the total curtilage area, again excluding the ground occupied by the original house.
This is a whole-property calculation, not a separate allowance for each project.
It can include:
- later extensions to the original house;
- existing and proposed garages;
- sheds and other outbuildings;
- separate detached buildings, including in some cases ones that were built at the same time as the original house.
The original dwellinghouse itself is excluded from both sides of the calculation.
Driveways and ordinary patios don’t generally count simply because they cover ground, because the Class A restriction is concerned with ground covered by buildings. Other structures can require closer consideration depending on what has actually been constructed.
And, importantly, a building that has genuinely been demolished does not continue to cover ground for the current 50% calculation.
So although the rule is often described as “you can build on half your garden”, that’s an unreliable shorthand. The statutory calculation concerns buildings within the property’s curtilage and excludes the footprint of the original dwellinghouse.
Five things people commonly get wrong
“It’s under three metres, so it’s automatically allowed.” Depth is only one condition among several. Height, boundary proximity, position, previous extensions, designated land status and the 50% rule can all independently prevent a project qualifying.
“My neighbour built the same thing.” Permitted development is assessed against each property’s own circumstances — including the original house, previous extensions, site position and any Article 4 Direction or planning condition — not simply against what a neighbouring house was able to do.
“The builder says it’s PD.” A builder’s experience is genuinely useful, but it isn’t a formal determination of planning law. Where the planning position matters, check the actual rules rather than relying solely on somebody’s experience of similar projects.
“The previous extension doesn’t count because I didn’t build it.” If it still exists and is relevant to the proposed enlargement, it can count. Permitted development does not give each new owner a fresh set of dimensions.
“It’s in the back garden, so planning doesn’t matter.” Location affects which rules apply — rear extensions are generally treated more permissively than extensions forward of the principal elevation — but “back garden” on its own guarantees nothing.
When to get formal confirmation
Where a project sits comfortably within the limits above, with a straightforward planning history and no Article 4 Direction, planning condition or designation complicating the position, some homeowners proceed on the basis of their assessment of the permitted development rules.
Where the position is borderline — an old extension whose original dimensions aren’t clear, a complicated stepped rear wall, a combined side-and-rear extension, a plot approaching the 50% threshold or uncertainty over whether rights have been removed — a Lawful Development Certificate can provide a formal decision on whether the proposed work would be lawful.
Our dedicated article on that decision covers when the cost is worth paying and when it generally isn’t.
Different rules apply outside England
The limits set out above apply in England.
Wales, Scotland and Northern Ireland each operate their own householder permitted development regimes under separate legislation, and the dimensions, definitions and conditions should not be assumed to match the English rules.
If the property isn’t in England, check the equivalent national guidance rather than applying any of the measurements above.
The real takeaway
Size alone never determines whether an extension is permitted development.
Position, height, boundary relationships, the property’s existing extensions, designation status and the cumulative amount of development on the plot all matter together — which is exactly why “how big can I go?” is a question best answered against the specific property, not against a number remembered from a forum post or a neighbour’s project.
