Do I need planning permission for a house extension?

Many single-storey extensions can be built without a full planning application, under what are called permitted development rights. Many others cannot: because of their size, because the house is a flat, or because it sits in a conservation area or under an Article 4 direction. This guide explains how to tell which applies to you, how the national rules work, and how councils across South West London look at extensions when an application is needed.

Two routes: permitted development or planning permission

Permitted development rights are a national grant of planning permission for certain types of work on houses, set out in government regulations. If an extension stays within the limits and conditions, you do not need to apply to the council for planning permission. If it goes beyond them, or if the rights do not apply to your property, you need a householder planning application.

Either way, the work still needs Building Regulations approval, and if it affects a shared wall or digs near a neighbour's foundations, you will usually need to serve notice under the Party Wall etc. Act 1996. Planning permission is only one of the approvals involved.

When permitted development does not apply

Before looking at sizes, check whether permitted development applies to your home at all. It does not apply to flats or maisonettes, so any extension to a flat needs planning permission. It is also restricted on houses in conservation areas, where, for example, side extensions and cladding need permission, and it can be removed for particular changes by an Article 4 direction. Listed buildings need listed building consent for most work, whatever the size.

Rights can also have been removed by a condition on an earlier planning permission, which is common on newer estates, so it is worth checking the planning history of your house. Your council's online planning register will show it.

Article 4 directions are more common than many people expect. On the Totterdown Fields and Heaver estates in Tooting, and on many roads in Merton Park, they remove permitted development rights for changes facing the street. In Richmond and Hammersmith & Fulham, borough-wide Article 4 directions mean every basement needs planning permission. Our area pages list the directions that apply locally.

The national limits, in outline

At the time of writing, a single-storey rear extension to a house can be built under permitted development if it extends no more than 3 metres beyond the original rear wall of a semi-detached or terraced house, or 4 metres for a detached house, and is no more than 4 metres high. If it is within 2 metres of a boundary, the eaves can be no higher than 3 metres. A larger single-storey rear extension, up to 6 metres or 8 metres respectively, is possible through a prior approval process, in which the council consults your neighbours before deciding.

Side extensions must be single storey, no more than 4 metres high and no wider than half the width of the original house. Two-storey rear extensions have tighter limits, including staying at least 7 metres from the rear boundary. Across all extensions, materials should be similar to the existing house, and extensions and outbuildings together must not cover more than half of the garden around the original house.

"Original house" matters here: it means the house as first built, or as it stood on 1 July 1948 if it is older. Earlier extensions count towards the limits, even if a previous owner built them.

These rules are detailed, with conditions not covered here, and they change from time to time. Always check the current position on the Planning Portal, which has an interactive guide for householders.

Get a Lawful Development Certificate

If your extension relies on permitted development, you do not have to tell the council before you build. It is still worth applying for a Lawful Development Certificate. It is the council's formal confirmation that the extension is lawful, it removes any doubt about measurements or conditions, and buyers' solicitors will often ask for it when you come to sell. It is a simpler, cheaper application than full planning permission, and you apply through the Planning Portal.

When you need planning permission: what councils look for

If your extension needs planning permission, the council will judge it against its local plan and design guidance. Most councils publish householder guidance, and although the details differ, the themes are consistent: the extension should look subordinate to the house, respect its neighbours' light and outlook, and leave a reasonable garden.

Some councils put figures on this. Hammersmith & Fulham's guidance says a rear extension should not normally project more than 3.5 metres beyond the original back addition, come within 4 metres of the rear boundary, or cover more than half of the original rear garden, which shapes most extensions in Fulham. Richmond's guidance treats single-storey rear extensions as usually acceptable up to around 3 metres on a terrace, 3.5 metres on a semi-detached house and 4 metres on a detached house, as described on our Richmond page. Sutton's guidance, which covers part of Worcester Park, sets similar depths.

These figures are what a council is likely to accept through a planning application. They are not the same as the permitted development limits, and one does not override the other.

What your neighbours can do

When you apply for planning permission, the council notifies neighbours and invites comments. Only planning matters count, such as loss of light, privacy or outlook, or an extension being overbearing; objections about property values or private disputes are not planning considerations. Under the prior approval process for larger rear extensions, adjoining neighbours are notified directly, and if any object, the council assesses the effect on their amenity before deciding. Talking to neighbours early, and showing them the drawings, often avoids objections altogether.

Pre-application advice

For anything unusual, such as a large extension, a house in a conservation area or a listed building, most councils offer paid pre-application advice. A planning officer reviews your proposals and tells you how they are likely to be received before you submit an application. It costs time and money, but it can save a refused application and a redesign.

How long it takes

Councils aim to decide householder applications within eight weeks of accepting them as valid, and prior approval applications for larger rear extensions have their own shorter timescale. Lawful Development Certificates usually take a similar time to a householder application. Party wall notices and building control can run alongside, so the planning route rarely needs to hold up the whole project if it is started early.

A practical checklist

Before designing an extension, check whether your home is a house or a flat, whether it is listed, whether it is in a conservation area or covered by an Article 4 direction, and whether earlier permissions removed any rights. Then compare the extension you want with the permitted development limits and your council's guidance. If it fits permitted development, get a Lawful Development Certificate; if not, consider pre-application advice before a full application. Our house extensions page explains how we help with approvals as part of a project.