Hi
I went in to this website, which was very informative on costs (how you kit it out is something else). It’s something we’re considering
Single storey extension
If it all depends, how much does a single storey extension cost?
In 2019, an estimate for the cost of a single storey extension is anywhere between £1,350 and £1,950 per m2.
In London and the South East, this could be anywhere between £1,800 and £2,300+ per m2. These prices are for for a good quality extension. Basic quality would naturally cost less and premium would be more.
Let’s say you’re looking for a 5m x 5m extension. The cost of an extension might be around £32,000. In London, maybe even up to £50,000+. That’s just for building the extension.
If we add on 10-15% for professional fees for the architect, planning application, building regulations, structural engineer and so on the cost goes up.
And let’s not forget about VAT.
All in all, a single storey extension could cost £40,000 at the lower end. £67,000 in London at the upper end.
householdquotes.co.uk/cost-of-extension/