Larger home extension prior approval or standard limits

You can build a single-storey rear extension 3 metres deep on a semi-detached or terraced house, or 4 metres on a detached house, without telling the council anything. Double those depths to 6 and 8 metres and you can still avoid a full planning application, but only through the larger home extension prior approval route, which puts your drawings in front of your neighbours and gives them 21 days to object. The trade is depth for certainty.

Both routes sit in Class A of Part 1 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015. Neither is available on designated land.

The standard depth limits under Class A

Within the ordinary Class A limits, a single-storey rear extension may project 3 metres from the original rear wall of a semi-detached or terraced house, or 4 metres from a detached house. Maximum height is 4 metres. Within 2 metres of a boundary the eaves height must not exceed 3 metres.

Other limits apply at the same time. No more than half the land around the original house may be covered by buildings, the extension must not be higher than the highest part of the existing roof, and materials must be similar in appearance to the existing house.

"Original house" means the house as first built, or as it stood on 1 July 1948 if earlier. Previous extensions count against the allowance, including those built by a previous owner, which is why the deeds and historic aerial imagery matter more than the current owner's memory.

No notification, no fee, no waiting. That is the attraction, and for a kitchen and dining space across the back of a house, 3 metres is frequently enough.

Depths available through prior approval

The larger home extension provisions double the permitted depth: 6 metres for a semi-detached or terraced house, 8 metres for a detached house. The extension must be single storey, no more than 4 metres high, and the eaves limit within 2 metres of a boundary still applies.

What you give up is the automatic right. You must notify the authority before starting, and you must not start until either the council tells you prior approval is not required, or the determination period has expired without a decision.

The notification has to include a written description of the proposed development including the dimensions, a plan showing the site and the proposed development, the addresses of adjoining premises, and your contact details. A fee applies, set by the national fee regulations and indexed, so check the current fee schedule before sending it.

How the neighbour consultation scheme works

The council notifies the owners and occupiers of adjoining premises, which means properties sharing a boundary, including at the rear. They are given 21 days to object.

The assessment is narrow. Where objections are received, the authority considers the impact of the extension on the amenity of adjoining premises, and nothing else. Design, materials, overdevelopment and the effect on the street are outside the scope.

That narrowness cuts both ways. An objection about the loss of a view, or about building work generally, carries no weight. An objection about loss of light to a kitchen window on a boundary carries a great deal.

What an objection does to the application

One objection from an adjoining neighbour moves the application from administrative to judgemental. The officer visits, looks at the relationship between the extension and the neighbour's windows and garden, and decides.

The arguments that succeed for an applicant are evidential. A section drawing through the boundary showing the extension against the neighbour's window. A daylight assessment prepared to the recognised building research guidance where light is the issue, which is what a Daylight and Sunlight Assessment covers. An Overshadowing Study where the complaint is about the garden rather than the house.

Offering a reduction is often better than arguing. A 6 metre extension reduced to 4.5 metres, with the boundary wall stepped down, can turn a refusal into an approval, and 4.5 metres is still beyond the standard limit.

Where prior approval is refused, you can appeal, and the householder appeal route is quicker than the standard one, as our comparison of the householder appeal and the fast-track commercial route explains. You can also simply build to 3 metres, which needs nothing.

Timescales and the deadline for a decision

The authority has 42 days from receipt of a valid notification to make its decision. If that period passes without a decision, you may proceed with the development as notified.

Three timing points matter:

1. The 42 days run from receipt of a valid notification, so missing information restarts the clock.
2. You must not begin before the period expires or the decision arrives, whichever comes first.
3. The development must be completed as notified. Building something different is not permitted development, and it is not covered by the prior approval either.

Against that, the standard 3 or 4 metre route needs no waiting at all. On a project where the builder is booked and the kitchen is ordered, six weeks of uncertainty has a real cost.

Designated land where the route is unavailable

The larger home extension route is not available on article 2(3) land, which covers conservation areas, National Landscapes, National Parks, the Broads and World Heritage Sites. On that land the standard Class A limits also tighten: rear extensions are limited to a single storey, and side extensions and cladding are removed.

Article 4 directions go further and can withdraw householder rights altogether in a defined area. Check the council's register of directions for the address before designing anything, because a direction makes an otherwise permitted extension an application.

For anything marginal, a Lawful Development Certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that proposed works are lawful without permission. It costs a fee and takes eight weeks, and it is worth both where a sale or a mortgage may depend on it later.

Choosing between certainty and depth

Ask three questions in order.

Does 3 or 4 metres give you the room you actually want? Measure it on the plan with furniture. A deeper extension often adds a darker middle rather than a better room, because the roof light arrangement has to work harder.

Is the boundary relationship comfortable? If the neighbour's main habitable window faces your boundary at close range, a 6 metre extension is likely to attract an objection you will lose. If the neighbour's own extension already projects a similar distance, the position is far easier.

Can you absorb six weeks? Where the answer is no, the standard route wins by default.

My own view, having watched both play out: on a terrace with tight boundaries, take the 3 metres and spend the saved contingency on the glazing and the roof. On a detached house with space either side, the 8 metre route is usually worth the wait.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. Enter your postcode on PlanScope to see which reports your project is likely to need, and draft a Planning Statement if you end up applying for permission instead. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.