Currently we recommend allowing between £2,500 and £3,000 per square metre for side extensions, rear extensions or front extensions.

For basement extensions, which are the most expensive way to add space to your home, we recommend allowing between £3,000 and £3,500 per square metre.

When it comes to roof extensions, our experience shows that it is better to allow a lump sum of:

  • £40,000–£60,000 for loft conversions with dormer windows
  • £80,000–£120,000 for mansard roof extensions
These ranges are based on averages from numerous actual projects that we have completed in and around London since 2007. We only use these rough costs per square metre during the early stages of design. This helps to maintain awareness of your target budget until a cost professional is appointed and more accurate estimates are provided.

The actual cost for your project will depend on many factors, including:

  • The complexity of the design
  • Level of structural support required
  • Selection of fixtures, fittings and finishes
The estimated costs per square metre mentioned above are for build costs only and exclude VAT and professional fees.