Class A rear extension depth on a detached house
Class A allows a 4 metre single-storey rear extension on a detached house, measured from the original rear wall, and up to 8 metres through the prior approval route.
A detached house gets 4 metres of single-storey rear extension under Class A permitted development, measured from the rear wall of the original house. Semi-detached and terraced houses get 3 metres. That extra metre is the only place in Part 1 where being detached buys you more depth, and it is why the question is worth asking specifically rather than reading a general guide to extensions.
Go beyond 4 metres and you are not automatically into a planning application. The larger home extension route allows up to 8 metres on a detached house, through a prior approval procedure that gives your neighbours a formal say.
The depth limits set by Class A
Class A of Part 1 of Schedule 2 to the General Permitted Development Order 2015 sets the rear extension limits. For a single-storey rear extension on a detached dwellinghouse the limit is 4 metres beyond the rear wall of the original dwellinghouse, with a maximum height of 4 metres.
"Original" does the heavy lifting in that sentence. It means the house as it stood on 1 July 1948, or as first built if later. A 1990s conservatory already taking 3 metres off your allowance leaves 1 metre, not 4. The allowance is cumulative across the life of the building and it does not reset when the house changes hands.
For an extension of more than one storey the rear limit drops to 3 metres, and it must be at least 7 metres from the boundary opposite the rear wall. That 7 metre figure rules out two-storey rear extensions on most suburban plots with shallow gardens.
Two limits apply whatever the depth. The extension must not be forward of the principal elevation fronting a highway, and buildings and structures in the curtilage, excluding the original house, must not cover more than half the area of the curtilage.
Larger extensions under the prior approval route
The larger home extension scheme, now a permanent part of Class A, doubles the depth: 8 metres for a detached house and 6 metres for any other house, single storey only.
The procedure, set out in paragraph A.4, works like this.
- Send the council a written description of the proposed extension with its dimensions, a plan showing the site and the proposed development, the addresses of adjoining premises, and your contact details
- The council notifies the owners and occupiers of adjoining premises, meaning those sharing a boundary with your property, and allows 21 days for them to comment
- If no adjoining occupier objects within that period, the council cannot refuse on amenity grounds and you may build once the period and the council's own 42 day window have run
- If an adjoining occupier does object, the council must consider the amenity impact on all adjoining premises and decide whether to grant prior approval
- You may not start before you receive the council's decision or 42 days have passed from a valid notification, whichever comes first
- On completion you must tell the council the development is finished
The 42 day period is a hard limit, and it runs from a valid submission. An incomplete notification restarts it, so include the adjoining addresses and a scaled plan first time.
One point that surprises people: only adjoining premises are consulted. The house three doors down, or across the road, gets no say under this route, however strong its view.
The larger scheme is not available on article 2(3) land, which means conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites.
Height limits at the eaves and the ridge
Depth is only one of four dimensions Class A controls, and height is where otherwise compliant designs fail.
The height of the building as extended must not exceed the height of the highest part of the roof of the existing house. The eaves height of any part within 2 metres of a boundary of the curtilage must not exceed 3 metres. A single-storey rear extension must not exceed 4 metres overall.
Measuring the 3 metre eaves limit catches people out because it is measured from the natural ground level at that point, not from your new patio or from the damp proof course. On a sloping plot the same extension can comply at one end and breach at the other, and the detail of how that measurement works is set out in the piece on measuring height under Part 1 against adjacent ground.
A flat roof with a parapet is measured to the top of the parapet.
Materials of similar appearance
The conditions in paragraph A.3 require the materials used in the exterior work to be of a similar appearance to those in the exterior of the existing house. Similar, not identical. A brick of close colour and texture is fine; a render finish on a brick house is not.
Rear elevations get more latitude in practice than front ones, and large glazed elements in a rear wall rarely cause a problem because the condition bites on the materials rather than the proportion of glass. Zinc, timber cladding and dark render on a brick house are the three finishes most often challenged.
Where an upper-floor window is formed in a side elevation, it must be obscure-glazed, and non-opening unless the openable part is more than 1.7 metres above the floor of the room.
Where the house has already been extended
Work out the remaining allowance before the design, not after.
Look for a conservatory, an existing single-storey rear addition, an outrigger that was added rather than original, and any rear infill between an original back addition and the boundary. All of them count against the 4 metres. Garages and outbuildings do not count against the depth figure. They do count towards the 50 per cent curtilage calculation.
Evidence matters here. The council will want to see the original footprint, which usually means an historic Ordnance Survey extract, old deeds plans, or an aerial photograph, together with a measured survey of what is there now. Where you cannot establish the original footprint, the safe assumption is the less generous one.
An extension that breaches Class A is unauthorised development, and the immunity rules have changed. Operational development was historically immune after four years, with the Levelling-up and Regeneration Act 2023 moving England towards a single ten year period, so check the current position rather than relying on the four year rule, as discussed in the piece on enforcement time limits for permitted development breaches.
Article 2(3) land and Article 4 directions
On article 2(3) land you keep the 4 metre single-storey rear allowance on a detached house, but you lose the larger home extension route, side extensions under Class A, and cladding under Class A.
Separately, a council can make an Article 4 direction withdrawing some or all of Class A in a defined area. These are common across conservation areas and some interwar estates, and they are site-specific: two streets apart, the position can differ. The council's planning policy pages list directions in force, and the process by which they are made is covered in how a council makes an Article 4 direction.
Check both before you measure anything. A direction can also be limited to particular elevations, so read the direction itself rather than a summary.
Confirming the position with a certificate
Permitted development is not a permission. Nobody signs anything, nobody checks your drawings, and a builder's assurance carries no legal weight.
The only formal way to confirm that works are lawful is a lawful development certificate. Apply under section 192 of the Town and Country Planning Act 1990 for proposed works, or section 191 for works already done. You submit drawings and a short statement setting out which limbs of Class A the scheme meets, the council determines it within eight weeks, and the certificate is a document a conveyancing solicitor will accept.
For a 4 metre extension on a detached house where the original footprint is clear, most people proceed without one. Where the house has been extended before, where ground levels slope, or where you intend to sell within a few years, the certificate fee is the better bet. A short Planning Statement setting out the measurements against each limb of Class A makes the application quicker to determine.
Enter your postcode on PlanScope to check which documents your extension is likely to need.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
How deep can a single-storey rear extension be on a detached house?
Four metres beyond the rear wall of the original dwellinghouse under Class A, with a maximum height of 4 metres. Any other type of house is limited to 3 metres. The depth is cumulative, so earlier rear additions and conservatories reduce what is left, and the original house means the building as it stood on 1 July 1948 or as first built if later.
How does the 8 metre larger extension route work?
You notify the council with dimensions, a site plan and the addresses of adjoining premises. The council consults adjoining owners and occupiers for 21 days. If nobody objects you can proceed once the council's 42 day period has run; if someone objects the council assesses amenity impact and decides whether to grant prior approval. It is unavailable on article 2(3) land.
Does a conservatory count against my 4 metres?
Yes, if it extends beyond the rear wall of the original house. The Class A depth allowance is cumulative over the life of the building and does not reset on sale, so a 3 metre conservatory leaves only 1 metre. Enclosed and unheated construction makes no difference to the planning calculation.
Where is the 3 metre eaves height measured from?
From the natural ground level at the point where the measurement is taken, for any part of the extension within 2 metres of a curtilage boundary. New paving, a raised patio or made-up ground do not raise the datum. On a sloping plot a single extension can comply at one end and breach at the other.
Do I need a certificate for a permitted development extension?
It is not compulsory, but a lawful development certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that works are lawful. It is worth having where the house has been extended before, where ground levels vary, or where you expect to sell within a few years.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.