We are building a new Extension Cost Calculator!

Sorry it is not ready yet, but if you’re thinking of extending your house, this free tool will estimate how much it is likely to cost. Answer a few simple questions, including where you live, the kind of extension you’d like to add, and how much of the work you’d like to do yourself, and it will provide you with an instant, accurate result as to how much your finished project should cost.

sloping site bungalow remodelled with a modern extension

How much will an extension cost?

There are many factors involved in costing an extension project, from the size of the project right down to the quality of the paint you use on the walls. For a basic breakdown of average extension prices, follow these guidelines:

  • From £1,500 per m² for basic construction work, up to £3,000 per m² for high-spec materials
  • If you’re adding bi-fold doors, allow £1,400 to £1,800 per metre for these
  • Small extensions, under 15–20 square metres, have higher costs per square metre
  • Clay, peat, lots of nearby trees and sloping ground will all increase foundation costs
  • As a rough guideline, 50% of the budget will cover construction whilst the other 50% will cover the interior

You must approach several builders and get full quotes for the proposed extension. Each builder will have different circumstances that will effect their quote, even though the materials and work won’t change. Approaching only one or two builders could give you an idea of a false average building cost.

If you would like to use our new calculator, fill in your details below and we will let you know when it’s ready to use.