The cost of a house extension

Many families start thinking about an extension when the house begins to feel small.

The building work is the biggest part of the bill. Several other costs come with it, and most of them depend on the size and shape of what you build.

These notes cover houses in England.

The size and shape you choose

Many extensions can go ahead under permitted development rights, without a planning application. Those rights set limits on size.

A single-storey rear extension can usually reach 4 metres beyond the original rear wall of a detached house. For other houses the limit is 3 metres. Through a process called prior approval, the limits rise to 8 metres and 6 metres. The council then asks the neighbours for their views before deciding.

An extension of more than one storey can reach 3 metres beyond the rear wall at most. Side extensions of more than one storey need planning permission. The "original house" means the house as it was first built, or as it stood on 1 July 1948. Extensions a previous owner added count towards the limits.

Planning permission

A design that goes beyond the limits will usually need a householder planning application. Flats, maisonettes and some houses in conservation areas follow different rules. Your local planning authority can tell you whether your project needs permission.

Councils can serve an enforcement notice on work that needed permission and did not have it. The notice can order the owner to undo the changes.

An extension that adds 100 square metres or more of internal floor space may be liable for the Community Infrastructure Levy. Some projects qualify for relief or exemption. Claims have strict timing rules, so ask the council early.

Building regulations

The Building Regulations 2010 cover the extension of buildings. You might need building regulations approval as well as planning permission.

Homeowners can apply to the council's building control team or to a private registered building control approver. Councils base their fees on their costs. The total floor area of an extension is one of the things that sets the fee. Private approvers agree their fees with you directly.

With a full plans application, you can expect a decision within 5 weeks, or 2 months if you agree. A completion certificate usually follows within 8 weeks of the work finishing. You may need the certificates when you come to sell the house.

Party walls and the neighbours

Building on or at the boundary, or digging near a neighbour's foundations, means giving your neighbour notice. The notice goes between 2 months and a year before work starts.

Your neighbour has 14 days to reply in writing. If the two of you cannot agree, surveyors draw up a party wall award. It says what work happens, how and when, and who pays for each part, including the surveyors' fees. You cannot act as your own surveyor.

Choosing a builder