Do you need
planning permission
for an extension?

What you could build

On a typical 5-metre-wide terraced house, a 3-metre rear extension gives you roughly 15 square metres of additional space. That's enough to open up the kitchen entirely, bring in natural light through floor-to-ceiling glazing, and create the kind of ground floor where cooking, eating, and living all happen in one generous, connected space - with proper access to the garden.

On a wider semi-detached or detached house, the possibilities are bigger. A side return extension reclaims that narrow strip of dead space between the house and the boundary - even a metre of additional width running the full depth of the ground floor can transform a cramped galley kitchen into a room that actually works. A deeper rear extension on a detached house - up to 4 metres under standard permitted development, or up to 8 metres through prior approval - can fundamentally change the scale and feel of the home.

All of this is possible without a planning application. The question is whether your specific project meets the conditions.

01The rules: how far can you extend?

Class A, Part 1, Schedule 2, GPDO 2015 — A.1(f)

These are measured from the outside face of the original rear wall - the house as it was first built, or as it stood on 1 July 1948 if built before then. Not from any existing extension.

02Larger extensions through prior approval

Class A, Part 1, Schedule 2, GPDO 2015 — A.1(g)

There is a route that allows deeper single storey extensions: up to 6 metres for semi-detached and terraced houses, and up to 8 metres for detached houses. This is sometimes called the "larger home extension scheme" and was made permanent in May 2019.

Prior approval is not a full planning application. You submit a description and site plan, the council writes to your immediate neighbours, and if no one objects within 21 days, approval is either confirmed or deemed granted after 42 days. The current fee is £249.

However, if a neighbour does object, the council must assess the impact on their amenity. In dense areas like London, a 6-metre rear extension on a mid-terrace house with neighbours on both sides is unlikely to proceed unchallenged. On a detached house with more distance from neighbours, the route is more straightforward.

03Two storey extensions

Class A, Part 1, Schedule 2, GPDO 2015 — A.1(h) to A.3(c)

Two storey rear extensions have a maximum depth of 3 metres for all house types. There is no prior approval route - the 3-metre limit is absolute. They must be at least 7 metres from the rear boundary, and the roof pitch must match the existing house as closely as practicable.

On a typical London terraced house with a 10-metre garden, the 7-metre boundary rule means your two storey extension can only project 3 metres before hitting the constraint - which happens to match the depth limit anyway. On shorter gardens, the boundary becomes the limiting factor before the depth rule does.

04Height limits

Class A, Part 1, Schedule 2, GPDO 2015 — A.1(c) to A.1(i)

No extension can exceed the height of the highest part of the existing roof. Single storey extensions have an additional absolute maximum of 4 metres.

The eaves of the extension - the point where the roof meets the top of the wall, where the guttering sits - must not exceed the eaves of the existing house. And if any part of the extension is within 2 metres of a boundary, the eaves are further capped at 3 metres. This boundary rule catches a lot of people, particularly on narrow plots.

05Side extensions

Class A, Part 1, Schedule 2, GPDO 2015 — A.1(e) to A.2(b)

A side extension cannot be wider than half the width of the original house. It must be single storey only and must not extend forward of the front wall.

In designated areas - conservation areas, National Parks, AONBs, World Heritage Sites, and the Broads - side extensions are not permitted development at all. If your property is in a designated area, a side extension will need a planning application.

06Materials

Class A, Part 1, Schedule 2, GPDO 2015 — A.3(a)

The exterior materials must be "of a similar appearance" to the existing house. Similar means similar in colour, texture, and visual character - not just the same material type. A red brick house needs a red brick extension, not a black brick one and not a timber-clad one. The test is visual: would a reasonable person looking at the extension say the materials match the house? An experienced designer can work creatively within the word "similar" - particularly where the extension is at the rear and less visible from the street - but a noticeably different finish risks falling outside permitted development.

07The 50% curtilage rule

Class A, Part 1, Schedule 2, GPDO 2015 — A.1(b)

All buildings on your plot - extensions, outbuildings, garages, sheds, everything except the original house itself - must not cover more than half the total land area. This is called the curtilage limit. It is cumulative and includes structures built by previous owners.

08What always needs planning permission

Class A, Part 1, Schedule 2, GPDO 2015 — A.1(e) to A.1(k)

Front extensions - any extension forward of the principal elevation - always require planning permission. If your property is a flat, a maisonette, a listed building, or was originally a commercial building converted to residential use, permitted development rights do not apply.

Balconies, verandas, and raised platforms higher than 300mm above ground level are also excluded. To be clear: this means a raised balcony or roof terrace is never permitted development, but a low timber deck in your garden at or near ground level (300mm or below) is not caught by this restriction.

09Conservation areas and designated areas

Class A, Part 1, Schedule 2, GPDO 2015 — A.2(a) to A.2(c)

If your house is on article 2(3) land - conservation areas, National Parks, AONBs, World Heritage Sites, or the Broads - additional restrictions apply. Side extensions and cladding changes are not permitted development. Two storey rear extensions are prohibited. But single storey rear extensions within the standard depth limits can still qualify. Many homeowners in conservation areas extend their homes under permitted development - they just need to understand which conditions apply.

10Lawful Development Certificate

Town and Country Planning Act 1990 — Section 192

Permitted development means you don't need planning permission. But it is strongly advisable to apply for a Lawful Development Certificate from your council, confirming in writing that your extension is lawful. The current fee is £274 via planningportal.co.uk. Your solicitor will almost certainly ask for one when you sell.

One nuance most guides miss: unlike planning permission, which gives you 3 years to start work, a Lawful Development Certificate confirms lawfulness on the date it is issued only. If the rules change before your extension is complete, the certificate may no longer apply. Start work promptly once you have one.

For detailed visual guidance on measurements and conditions, see the government's official technical guidance (pages 4-12 cover rear extensions).

Related guides

Conservatory planning permission - conservatories follow the same rules as extensions, with one key difference on materials.

Side extension rules - single storey only, no wider than half the house, and banned on designated land.

Two storey extension - the 7 metre boundary rule - why the rear boundary distance often constrains a two storey extension before the depth limit does.

Eaves height within 2 metres of a boundary - the 3 metre rule that catches people on narrow plots.

Prior approval for a larger extension - how to extend up to 6m or 8m through the prior approval route.

Previous extensions and your PD allowance - why previous work counts against your limits and doesn't reset when you buy.

The 50% curtilage rule - how to calculate whether your plot has space left for more building.

Balconies and raised decking - why balconies and raised platforms are never PD.

Permitted development in a conservation area - what changes when your property is on designated land.