The short answer is: anything you want to pay! The longer, more nuanced, answer is: it all depends.

Depends on your location, the availability of architects, builders and craftsmen (and women), as well as size of extension, the materials being used and the amount of glazing.

There are though some guideline figures:

We asked David Frazer for some ballpark figures in this corner of England:

Building work costs depend not only on where you live and the team you’ve hired, but on the results you’re after. Expect to pay around £1,500 to £1,900 per square metre for an extension of basic quality; £1,900 to £2,200 per square metre will give you a finish of good quality; and £2,200 to £2,400 per square metre will allow you to achieve excellent quality.

From the horse’s mouth then.

What this means to save you rooting for a calculator on your phone or in a drawer is that a single storey extension of 4,m x 5m will cost:

£30,000 and upwards for an extension of basic quality

£38,000 for a good quality build

£44,000 for an extension with an excellent finish

Now these figures are not set in stone and it is wise to remember that professional fees are not included in this and they typically add 10% to 15% of costs (just watch an episode of Grand Designs on YouTube to see projected costs and actual costs).

Architects’ fees for extending a house are usually between 3 and 7%  of the total construction cost.

You also need to factor in:

  1. Structural engineers’ fees
  2. Surveyors’ fees
  3. Planning fees
  4. A certificate of lawful development
  5. A request for discharging planning conditions
  6. Building control charges
  7. A party wall agreement (if needed)
  8. Project management costs
  9. VAT at 20% of the labour, materials and services

The best way though for anyone who is contemplating a single storey extension to do is call Envision CAD on 01603 273623 or email David at

You can also use our handy contact form or message us on Facebook, Instagram or LinkedIn.