two storey extension with ground floor decked terrace

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.


Related articles: The ultimate guide to extending your home | 10 ways to cut the cost of your 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.

To get a more comprehensive cost breakdown, start your calculation


How does the extension cost calculator work?

The extension cost calculator is based on the very latest costs for materials and labour, and will provide you with cost estimates for:

You can then fine-tune your estimate to match your needs, adding everything from your dream kitchen, bathroom, wet room or staircase, to the perfect bi-fold doors and statement windows. It will also advise you on the cost of essentials, like groundworks and utilities.

You can either start a new calculation or amend an existing one – receiving an estimate for your project in seconds.

Start estimating now

Whatever kind of extension you’re thinking of adding, use our free tool to find out whether you’ll need to apply for planning permission.

Do I need planning permission for my extension?