Class A rear extension depth limits by house type

The Class A rear extension depth limit is four metres beyond the original rear wall for a detached house and three metres for a semi-detached, terraced or end-of-terrace house. Both limits apply to a single-storey extension with a maximum height of four metres. A larger single-storey rear extension, up to eight metres for a detached house or six metres for any other house type, is possible under the prior approval procedure in the same Class, which requires notifying the council and a neighbour consultation period before work starts.

All of these figures come from Schedule 2, Part 1, Class A of the Town and Country Planning (General Permitted Development) (England) Order 2015, as amended. The limits are cumulative and unforgiving, and one breach takes the whole extension outside permitted development.

The depth limits and how house type changes them

House type is the first thing to settle, because it changes the depth by a full metre. A detached house is one that shares no wall with another dwelling. An end-of-terrace house is not detached, and neither is a semi-detached house, even one where the shared wall is only to a garage.

A two-storey rear extension has its own rules. The depth limit is three metres whatever the house type, and it must not come within seven metres of the boundary opposite the rear wall. That seven metre standoff removes the option on most modern plots with shallow gardens, which is why two-storey rear extensions so often need a full application.

Where the extension is more than one storey, the roof pitch of the upper storey has to match that of the existing house as far as practicable, and any upper-floor window in a wall or roof slope forming a side elevation must be obscure-glazed and non-opening below 1.7 metres above floor level.

Eaves and ridge heights and where they are measured from

Heights under Class A are measured from the highest part of the ground next to the building, not from the internal floor level or from the lowest point of a sloping garden. On a site that falls away from the house, that definition works in your favour. On one that rises, it does not.

The limits that matter:

• Maximum height of a single-storey rear extension: four metres.
• Maximum eaves height anywhere on the extension: three metres where any part of it is within two metres of a boundary of the curtilage.
• Maximum overall eaves height otherwise: the eaves height of the existing house.
• Maximum height of the extension: no higher than the highest part of the roof of the existing house.

The three metre eaves rule within two metres of a boundary is the single most common reason a rear extension design fails. A flat roof at 3 metres with an upstand and a parapet can exceed it. So can a pitched roof whose eaves sit at 3.1 metres because the internal floor was dropped. Measure it on the drawing, from the highest adjacent ground, before the design is fixed.

Measuring from the original rear wall

"Original" means the house as it was built, or as it stood on 1 July 1948 if it was built before that date. Any previous extension counts against your allowance, whoever built it and however long ago.

That has two consequences people find unwelcome. A 1990s conservatory occupying two metres of the original three metre allowance leaves one metre, even though conservatories are sometimes assumed not to count. And an outrigger or original rear projection on a Victorian terrace means the measurement is taken from the original rear wall of the main house in some configurations and from the rear of the projection in others, depending on which wall the extension actually extends beyond.

Where the position is unclear, the honest answer is that it needs checking against the specific plan form. Two neighbouring terraces with different original footprints can have different allowances.

There is also a curtilage cap that is easy to overlook. The total area of ground covered by buildings within the curtilage, excluding the original house, must not exceed 50 per cent of the curtilage. Outbuildings, sheds and previous extensions all count towards it.

Extensions within two metres of a boundary

Beyond the three metre eaves limit, proximity to a boundary brings no additional depth restriction under Class A. Where the plot is in a conservation area, a National Landscape, a National Park, the Broads or a World Heritage Site, though, the position changes materially. On that land, cladding of any part of the exterior is excluded, a rear extension of more than one storey is excluded, and side extensions are excluded altogether.

So a two-storey rear extension in a conservation area always needs a full application, and a single-storey rear extension there is still available within the standard depth limits.

Side walls, wrap-arounds and why they fail

A side extension under Class A must be single storey, no more than four metres high, and no wider than half the width of the original house. That half-width rule is measured against the original house rather than the house as extended.

A wrap-around extension, combining a rear and a side element in one continuous structure, is where most permitted development claims come apart. The Order treats the whole thing as a single extension, so the side extension restrictions apply to it. A wrap-around that extends three metres to the rear and also along the side is generally not permitted development, because the side element cannot satisfy both the rear depth allowance and the side width and single-storey rules at once. That does not make it unacceptable, it just means it needs a householder application. Many wrap-arounds are approved on their merits.

The same logic explains why flats and maisonettes have no Part 1 rights at all, and why commercial premises fall under quite different classes, as with extending a shop under Part 7.

Materials, and confirming the position formally

Class A requires the materials used in any exterior work to be of a similar appearance to those in the exterior of the existing house. Similar, not identical. A brick match that reads as the same brick from the street satisfies it; a rendered extension on a brick house probably does not, and a cladding change on article 2(3) land is excluded outright.

A local Article 4 direction can remove Class A rights entirely for householder development in a defined area, and that is separate from conservation area status. Check the council's Article 4 register for the address, not just the constraint layer.

The only formal way to confirm that works are lawful is a Lawful Development Certificate. A certificate of lawfulness of proposed use or development under section 192 of the Town and Country Planning Act 1990 gives you a decision you can rely on before building, and it costs a fraction of a householder application. For anything close to a limit, or where an earlier extension muddies the allowance, it is money well spent. A short Planning Statement setting out the measurements against each limit in Class A is the document that supports either route. Protected areas bring their own restrictions, set out class by class in Article 2(3) land permitted development.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it, and remember that a Lawful Development Certificate is the only formal way to confirm that works are lawful.