Extending your house

An extension adds rooms or space to the house you already live in.

Two sets of rules come into it before the building starts. Planning permission and building regulations approval are separate, and an extension may need both.

These notes follow the rules for houses in England. The rules in Wales may differ, and Scotland and Northern Ireland have their own.

Extensions without a planning application

Many house extensions in England can go ahead under permitted development rights, with no planning application. Every limit has to be met, and the limits count earlier extensions too.

The rules measure from the original house. That means the house as first built, or as it stood on 1 July 1948 if it is older. An extension built by a previous owner counts too.

The rights cover houses only. Flats, maisonettes and some converted houses follow different rules. Your council may also have removed the rights in your area with an Article 4 Direction.

The main size limits

A single-storey rear extension can reach 4 metres beyond the original rear wall of a detached house. For any other house the limit is 3 metres. It can be no more than 4 metres high.

A side extension can be one storey only, up to 4 metres high and up to half the width of the original house. An extension of more than one storey can reach 3 metres back. It must also stay 7 metres from the boundary behind it.

Extensions and other buildings can cover no more than half the land around the original house. The outside materials must look similar to the ones on the house. In a conservation area or a national park, the limits are tighter.

Larger rear extensions and prior approval

A single-storey rear extension can go further through a process called prior approval. The limit becomes 8 metres for a detached house and 6 metres for any other house.

You apply to the council, and the council asks your next-door neighbours for their views. If a neighbour objects, the council decides whether the effect on their home is acceptable.

For any extension, you can also apply for a lawful development certificate. A certificate for planned work confirms that the work would be lawful for planning purposes. Anyone can apply to the council for one.

Building regulations

Most extensions need building regulations approval, and that is separate from planning. You can apply to your council's building control team or to a registered private approver.

A full plans application is the most thorough route. The decision comes within 5 weeks, or 2 months if you agree. A building notice suits smaller projects, and work can start 2 days after it goes in.

When the work is finished and complies, you receive a completion certificate. GOV.UK points out that you may need these certificates when you come to sell.

Building near a shared wall

If the work is on or near a wall you share with a neighbour, you must tell them under the Party Wall etc. Act 1996. This applies in England and Wales. A party wall agreement is separate from planning permission and building regulations.