Class A rear extension limits and the larger home scheme

If your house is detached, Class A of the General Permitted Development Order lets you build a single-storey rear extension projecting up to 4 metres from the original rear wall without applying for planning permission. For a semi-detached or terraced house the figure is 3 metres. Either way the extension can be no more than 4 metres high, and where it comes within 2 metres of a boundary the eaves cannot exceed 3 metres. Those are the class A rear extension limits most people are looking for.

The larger home extension scheme doubles the depth, to 8 metres detached and 6 metres otherwise, in exchange for telling the council first and letting your neighbours comment. It is still permitted development. It is not a planning application.

The limits that apply every time

Class A sits in Schedule 2, Part 1 of the Order. Paragraph A.1 lists the circumstances in which the right does not apply, and every one of them has to be satisfied.

Two further limits catch people out. The extension cannot be higher than the highest part of the existing roof, and the eaves of the extended part cannot be higher than the existing eaves. And nothing can project beyond a wall forming the principal elevation, or a side elevation that fronts a highway.

The right also disappears entirely where the dwelling itself was created under Class M, MA, N, P, PA or Q of Part 3. A flat converted from an office under Class MA has no Class A rights at all.

Why measurements start further back than you think

Every figure above is measured from the original dwellinghouse. That means the house as it was first built, or as it stood on 1 July 1948 if it is older than that.

So if a previous owner added a 2 metre rear extension in 1995, that projection counts. On a semi-detached house you have 1 metre of Class A depth left, not 3. This is the single most common reason a householder discovers, mid-build, that the works were never permitted development, and it is worth reading what the original dwellinghouse means before ordering drawings.

Conservatories count. Outbuildings count towards the 50 per cent curtilage figure. A garage converted and extended counts.

The larger home extension route

Paragraph A.4 sets out the prior approval procedure. It applies only to a single-storey rear extension, only between the standard limit and the larger figure, and only at a maximum height of 4 metres.

The steps are fixed:

1. Write to the local planning authority before starting, with a description of the development including how far it extends beyond the rear wall, its maximum height and its eaves height.
2. Include a plan showing the site and the proposed development, together with the addresses of every adjoining property, including those to the rear.
3. Provide your contact address and an email address if you are content to be contacted that way.
4. Pay the notification fee, which is set nationally and revised periodically.
5. Wait. The council has 42 days from receipt of the valid notification.
6. If the council has not told you its decision within 42 days, the development may go ahead in accordance with what you notified.
7. Notify the council in writing when the work is finished.

The 42-day figure is worth pinning down, because several other prior approval classes run on a different clock, and the 56 day prior approval deadline used elsewhere in the Order does not apply here.

What the neighbour consultation actually covers

The council serves notice on the owners and occupiers of every adjoining property, describing the proposal and giving 21 days for representations. Adjoining means sharing a boundary, including at the rear and diagonally in some layouts.

If nobody objects, prior approval is not required and you can build. If anyone objects, the council must consider the impact of the proposed development on the amenity of adjoining premises, and only that. Loss of view is not amenity. Nor is a fall in property value, disruption during construction, or a boundary dispute.

What counts is daylight and sunlight to habitable rooms, overshadowing, outlook, and any sense of enclosure from a long flank wall running down a boundary. A 6 metre extension against a north-facing boundary wall in a narrow Victorian terrace is the case that gets refused. The same extension on a wide plot with a 4 metre side garden rarely is.

If the council refuses prior approval, you can appeal to the Planning Inspectorate, or apply for planning permission with a planning statement setting out the amenity case properly.

Materials, side extensions and two storeys

Three conditions in paragraph A.3 apply to the finished work. Materials must be similar in appearance to those of the existing house. Any upper-floor window in a side elevation must be obscure-glazed, and non-opening unless the opening part is more than 1.7 metres above the floor of the room. Neither is optional, and a mismatched brick is a real enforcement risk rather than a theoretical one.

Side extensions are tighter. A single-storey side extension must be no more than 4 metres high, no wider than half the width of the original house, and cannot have more than one storey.

Two-storey rear extensions are permitted development only where they project no more than 3 metres and sit no closer than 7 metres to the rear boundary. Most urban plots fail the 7 metre test, which is why almost every two-storey rear extension in a terraced street needed permission.

Article 2(3) land

Conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites are Article 2(3) land. There, Class A is cut back sharply. No cladding of the exterior. No rear extension of more than one storey. No side extension at all. And the larger home extension route is unavailable, so the 3 and 4 metre limits are the ceiling.

Article 4 directions go further still and can remove householder rights altogether in a defined area. They are made by the council and published on its website, and a property inside one needs a full planning application for work that would be permitted development two streets away.

Proving it afterwards

Permitted development produces no paperwork by itself, which becomes a problem at sale. 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, and applying before you build, while the drawings and the measurements are to hand, is far easier than reconstructing the case a decade later.

Keep the notification correspondence, the 42-day acknowledgement and the completion notice together with the certificate. That bundle answers almost every question a conveyancer will ask.

You can enter your postcode on PlanScope to check whether your property sits in a conservation area or another designation that changes the rules before you commit to a design.

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.