How much does an extension cost? Four groups of money, separated

How how much an extension costs works

1. Settle whether it is permitted development. A great many house extension are permitted development, which means no planning application, but the limits on depth, height, materials and how much of the plot is already built on are specific and a designer should read them against your own house before anybody prices anything.
2. Get drawings and a structural design. Existing and proposed drawings, then a structural engineer's calculations for whatever is being opened up or carried. On a house extension this is a small share of the total and it is what turns an idea into something three builders can price identically.
3. Serve the party wall notices in time. Work on or near a shared wall, or excavation within three or six metres of a neighbour's foundations, engages the Party Wall etc. Act and the notice periods are set in months rather than weeks. Serving late is the commonest reason a start date slips.
4. Price it against a specification, not a sketch. Ask each builder to price the same written specification, with provisional sums shown as provisional sums. Two quotes that differ by a third almost always differ in what is included: the kitchen, the flooring, the decoration and the drainage diversion are the usual absentees.
5. Agree stages, retention and building control. A staged payment schedule against completed work, a small retention until sign-off, and a clear statement of who is notifying building control. The completion certificate is what a future buyer's solicitor will ask for.

Insurance and paperwork

Common questions

Why do house extension prices differ so much between builders?

Because they are pricing different scopes. A quote with a provisional sum for the kitchen and one that excludes the kitchen entirely will look thousands apart while describing the same building work. Insist on the same written specification for each.

What is a provisional sum?

An allowance for work that is not yet specified, such as a kitchen or a bathroom. It is legitimate and it is also the line that moves most, so ask what the allowance assumes and what happens when the real choice costs more.

Should I use a contract?

For anything of this size, yes. A simple written contract with a specification, a staged payment schedule, a start and finish date and a retention protects both sides, and it is the document that decides what happens when something is found in the ground.

How much does an extension cost UK-wide?

It is four groups of money that only become comparable once they are separated: the shell, which is groundwork, structure, walls and roof; the envelope, which is glazing, insulation and weatherproofing; the services, which are drainage, heating and electrics; and the fit-out, which is where two quotes most often differ.

Registered contractors by region

• Registered roofers in the East Midlands postcode area
• Registered roofers in the East of England postcode area
• Registered roofers in the London postcode area
• Registered roofers in the North East postcode area
• Registered roofers in the North West postcode area
• Registered roofers in the Scotland postcode area
• Registered roofers in the South East postcode area
• Registered roofers in the South West postcode area
• Registered roofers in the Wales postcode area
• Registered roofers in the West Midlands postcode area
• Registered roofers in the Yorkshire and The Humber postcode area