Single-storey rear extension over 4 metres deep

A rear extension over 4 metres deep can still be permitted development, but only through the prior approval route and only on a detached house. On a semi or a terrace the standard allowance stops at 3 metres and the larger allowance takes you to 6. On a detached house the figures are 4 and 8.

That second set of numbers is the whole point of the larger home extension procedure. You are not applying for planning permission. You are telling the council what you intend to build and giving your neighbours 21 days to object.

When the deeper rear extension route applies

Class A of Schedule 2, Part 1 to the General Permitted Development Order 2015 allows a single-storey rear extension of up to 3 m on a semi-detached, terraced or end-terrace house, and 4 m on a detached house, without any notification at all.

The larger home extension procedure doubles those figures to 6 m and 8 m. It was introduced as a temporary measure and made permanent in 2019, so there is no longer a deadline by which the work must be finished.

Conditions that apply to the deeper extension:

• Maximum height of 4 m overall, measured from the natural ground level.
• Where any part sits within 2 m of a boundary, the eaves height must not exceed 3 m.
• The eaves must not be higher than the eaves of the existing house.
• Single storey only. A two-storey rear extension has its own, much tighter, allowance.
• Materials similar in appearance to the existing house.
• Together with all other buildings and additions, no more than half the area of land around the original house may be covered.

How the neighbour consultation scheme runs

The procedure has a fixed shape and the dates matter more than the drawings.

1. You submit a prior approval application to the council with a written description of the proposal, including how far it projects beyond the original rear wall, its maximum height and its eaves height, a plan of the site, and the addresses of adjoining premises.
2. The council notifies the owners and occupiers of adjoining premises, meaning any property sharing a boundary, including at the rear, and gives them 21 days to object.
3. If nobody objects within 21 days, the council must give prior approval or tell you that approval is not required, and you can build.
4. If anyone objects, the council considers the impact of the extension on the amenity of adjoining premises. That is the only issue it may consider.
5. The council has 42 days from receipt of a valid application to notify you of its decision. If it does not, you may proceed.

That fifth point is the safeguard, and it is worth diarising the day of receipt. Do not start work on the strength of an expired 42-day period without written confirmation of the receipt date, because a dispute about validity restarts the argument at your expense.

Measuring depth, height and eaves correctly

Depth is measured from the rear wall of the original house. Original means as it was on 1 July 1948, or as first built if later, so a 1990s conservatory that was itself an addition does not reset the starting line. A house that already has a 2 m rear extension has 4 m of the 6 m allowance left, not 6.

Height is measured from the highest part of the natural ground level adjacent to the extension, not from a lowered patio or a raised deck. Where the garden falls away, that distinction can cost you the 4 m limit.

Eaves height on the boundary side is the condition most often breached in construction rather than in design. A flat roof with a parapet and a fascia detail can push a nominal 2.9 m eaves over 3 m once the build-up is added. Draw the section, mark the 3 m line, and check it again on site.

Where the larger allowance never applies

The deeper rear extension is not available on article 2(3) land, which covers conservation areas, National Landscapes, National Parks, the Broads and World Heritage Sites. It also does not apply to flats and maisonettes, which have no householder permitted development rights, or where an Article 4 direction has removed Class A, or where a condition on an earlier permission has removed Part 1 rights.

Check the constraints layer and the council's list of Article 4 directions before doing anything else. On a newer estate, a condition on the original outline permission removing permitted development rights is common and easy to miss.

What to file if you exceed the limits

Beyond 6 m or 8 m, or above 4 m in height, or over 3 m at the eaves near a boundary, you are into a full householder planning application. That is a different test entirely, because the council can then consider design, character, trees, highways and everything else, rather than amenity alone.

The pack is location plan, block plan, existing and proposed plans, elevations and a section. A short Planning Statement setting the proposal against the council's householder design guidance is not usually a validation requirement but does answer the officer's first question. A Design and Access Statement is required only for work in a conservation area or affecting a listed building.

Where the extension is deep and the neighbouring house has a window close to the boundary, a Daylight and Sunlight Assessment run to BRE 209 turns the argument from adjectives into numbers. On a modest householder scheme that is often disproportionate, and a properly drawn section showing the 45-degree line does the same job for less.

The objections that stop a prior approval

Under the neighbour consultation scheme the council may consider only the impact on the amenity of adjoining premises. Objections about house prices, construction noise, parking, drainage or the loss of a view are outside the test, and a well-written prior approval submission can say so.

What does count is loss of light to a habitable room window, loss of outlook, and an overbearing or enclosing effect on the adjoining garden and rear elevation. The moves that answer those points are the same ones that answer a refusal on a full application: step the plan so full depth is kept away from the shared boundary, drop the eaves on that side, and pull the flank wall 300 mm to 500 mm off the boundary line.

Confirming lawfulness afterwards

Prior approval confirms the council will not stop the extension on amenity grounds. It does not certify that every other Class A limit is met, and it is the applicant who carries that risk.

If the work will be sold or remortgaged, apply for a Lawful Development Certificate. A certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that proposed works are lawful, and it is far easier to obtain before the extension is built than to argue about afterwards.

Enter your postcode on PlanScope to see which documents your extension is likely to need before you notify the council.

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.