Double storey extension cost: cheaper per square metre, more complex to build

How double storey extension cost works

1. Settle whether it is permitted development. A great many double storey 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 double storey 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 is second storey extension cost lower per square metre?

One set of foundations and one roof serve twice the floor area, so the cost per square metre falls even though the total rises. That is the single most useful thing to know when weighing a two storey against a larger single storey.

Is planning permission more likely to be needed?

Yes. Two storey rear extensions have tighter permitted development limits, including a maximum projection and a distance to the rear boundary, and side extensions at two storeys are generally outside permitted development altogether.

What about the existing roof?

A two storey extension usually ties into the existing roof, which means opening it, matching tiles and forming a valley or an abutment. Matching a weathered roof covering is difficult and the quote should say what happens if a match cannot be found.

How do I compare the cost of double storey extension UK quotes?

Ask each builder to price the same written specification, with provisional sums shown as provisional sums. On a two storey build the lines that differ most are the structure where it meets the existing house and the work to tie into the existing roof, so check that both are priced rather than assumed.

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