Rear extension depth: 3m, 4m, 6m and 8m limits explained

Rear extension depth rules under permitted development come down to four numbers. A single-storey rear extension may project 3 m from the original rear wall of a semi-detached or terraced house and 4 m from a detached one without a planning application. Under the larger home extension scheme those figures rise to 6 m and 8 m, but only after a prior approval process in which the council consults your neighbours. All four come from Schedule 2 Part 1 Class A of the Town and Country Planning (General Permitted Development) (England) Order 2015, and each is hedged with height, boundary and location conditions that catch more extensions than the depth figures themselves.

The four depth figures and which house type each applies to

"Detached" means detached as originally built. A semi whose neighbour was demolished is still treated by most councils as a semi, and the point is one to settle with a Lawful Development Certificate rather than assume. Depth is measured from the rear wall of the original dwellinghouse, and the extension must be single storey to use either figure. A two-storey rear extension is limited by paragraph A.1(h) to 3 m of depth on any house type, and is not permitted at all if it would come within 7 m of the rear boundary.

Article 2(3) land means conservation areas, National Parks, National Landscapes (the former Areas of Outstanding Natural Beauty), the Broads and World Heritage Sites. On that land the 6 m and 8 m route does not exist, and Class A also removes side extensions and cladding. Green Belt is not article 2(3) land, so a Green Belt house keeps the larger allowance.

Class A A.1(f) and how eaves and ridge heights bite

Paragraph A.1(f) sets the 3 m and 4 m depths and adds that the single-storey extension may not exceed 4 m in height. Height is measured from the highest part of the natural ground next to the extension to its highest point, so a flat roof with a parapet or a lantern counts to the top of the parapet or lantern.

Two other height rules apply whatever the depth. Under A.1(i), if any part of the extension is within 2 m of a boundary, its eaves may not exceed 3 m. And under A.1(c) and A.1(d) the extension's eaves may not be higher than the eaves of the existing house, nor its ridge higher than the highest part of the existing roof. On a bungalow with 2.4 m eaves, that eaves rule is often the real limit rather than the 4 m depth.

Depth also interacts with the 50 per cent rule in A.1(b): the total area of ground covered by buildings in the curtilage, excluding the original house, may not exceed half the curtilage. On a small terraced plot with an existing outbuilding, a 3 m extension can breach it. Materials must be of a similar appearance to the existing house under condition A.3, and there may be no veranda, balcony or raised platform.

The larger home extension prior approval route step by step

The 6 m and 8 m allowances in A.1(g) come with the procedure in paragraph A.4, known as the neighbour consultation scheme. It is not a planning application and the council cannot refuse it on design grounds. It can only refuse on the amenity of adjoining premises, and only if a neighbour objects.

1. Before starting work, send the council a written description of the extension, including its depth from the original rear wall, its maximum height and its eaves height, with a plan showing the site and the extension, the addresses of every adjoining property, and your contact details.
2. The council notifies the owners and occupiers of the adjoining premises, who have 21 days to object.
3. If nobody objects, the council must tell you in writing that prior approval is not required.
4. If an objection is received, the council considers the effect on the amenity of all adjoining premises, and either grants or refuses prior approval.
5. If the council has not given you any decision within 42 days of receiving your information, the development may proceed as if approval had been given.
6. Build in accordance with the details you sent, complete within three years of the prior approval date, and notify the council in writing when the extension is complete.

Check the current fee for the prior approval on the Planning Portal before you send it, because fees are indexed and the figure changes. If the council refuses, you can appeal to the Planning Inspectorate or scale the extension back to the standard 3 m or 4 m and proceed without consultation.

Where the depth allowance drops to zero

Several situations remove Class A entirely, whatever the depth.

• Flats and maisonettes have no Part 1 rights.
• Houses created by change of use under Part 3 Classes G, M, MA, N, P, PA or Q are excluded by A.1(a), so a barn conversion or a converted shop cannot use Class A.
• An Article 4 direction can remove Class A for an area, most often in conservation areas.
• A condition on an earlier planning permission can remove permitted development rights for that house, so check the decision notice for the original estate or for any past extension.
• A listed building needs listed building consent for an extension regardless of the planning position, so the depth rules are not the constraint there. A Heritage Statement will accompany that consent.

There is a subtler zero as well. Where the house has already been extended to the rear since it was built, the allowance is measured from the original wall and may already be used up. A 1970s rear extension of 3 m on a semi means no further permitted development depth, even though the current rear wall could take more.

Measuring from the original rear wall, not today's wall

"Original" is defined in Article 2 of the Order as the house as it was first built, or as it stood on 1 July 1948 if built before then. Every later addition, whether built with permission or under permitted development, sits on top of the original and eats into the allowance.

The rear wall is the wall facing the rear of the plot, and a house can have more than one. On an L-shaped house with a two-storey rear outrigger, the rear wall of the outrigger and the rear wall of the main house are both rear walls, and the extension may project the permitted depth from each. The government's technical guidance for householders illustrates this and is the document council officers use, so read it before arguing the point. Filling in the side return next to an outrigger, the classic Victorian terrace project, is a rear extension measured from the main rear wall, but it also lies beside the outrigger's side wall, and where the plot is on article 2(3) land the side extension prohibition can catch it.

Measure from the outside face of the original brickwork to the outside face of the new wall. Draw it on the plan with the original footprint shown dashed. If you cannot establish what was original, the council's planning history and old Ordnance Survey maps usually settle it, and the drawing package for a Planning Statement should include that evidence when the depth is close to the line.

Getting a Lawful Development Certificate to prove it

A Lawful Development Certificate is the only formal confirmation that an extension is permitted development. Apply under section 192 of the Town and Country Planning Act 1990 for a proposed extension, or section 191 for one already built. The council must decide on the facts and the law, not on planning merits, and the certificate protects you when you sell, because a buyer's solicitor will ask for it. The fee is set at half the equivalent planning application fee, so check the current figure on the Planning Portal.

For the application you need existing and proposed plans and elevations with the depth, eaves and ridge dimensions shown, a location plan with the site edged in red, and a short statement citing the Class A paragraphs the extension complies with. No Design and Access Statement is needed. Where the extension exceeds the permitted depth, or the house is excluded from Class A, the route is a householder planning application, where the council's design guide and the 45-degree test replace the numbers above.

Illustrative example: a 1930s semi has an original rear wall and a 1990s single-storey rear extension 2 m deep with permission. The owner wants a full-width extension 5 m deep from the original wall, flat roof at 3.2 m, 0.5 m from the side boundary. Standard permitted development is exhausted beyond 3 m. The larger home extension route allows 6 m, so the extra 3 m of depth beyond the existing extension is possible with prior approval. The eaves at 3.2 m, within 2 m of the boundary, breach the 3 m eaves limit, so the roof is lowered to 3 m at the eaves before the notice goes in. The figures are invented to show the checks.

PlanScope can draft the statement of compliance for a certificate application, or the Planning Statement for a full application, from your postcode and a photograph of the rear of the house, flagging any dimension that still needs confirming on site. Enter your postcode on PlanScope to see which reports 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.