"How much will my extension cost" is the first question on almost every enquiry we get, and until now the honest answer has always been "it depends, come and see us." That's still true for an accurate figure, but we wanted to give people a real starting point before they pick up the phone, so we've built a cost calculator that does exactly that.
What it's built from
The range it gives you starts from our actual rates, £2,200 to £2,500 + VAT per m² for a single-storey extension, with an additional £1,800 per m² for the upper floor on a double-storey extension. That's not a generic industry figure pulled from a national average, it's what we quote, adjusted for the kind of extensions we build across Bedfordshire, Cambridgeshire, Hertfordshire and Buckinghamshire.
What you can add to it
Beyond the base build, the calculator prices bifold doors, roof lights (manual or electric, since they cost differently), a whole-house rewire if your property needs one, and underfloor heating, all using ballpark figures rather than folding them invisibly into a single number. Roof type and underfloor heating are flagged as our own estimates rather than confirmed rates, since both genuinely vary a lot by design and specification.
It deliberately stops short of pricing a kitchen or bathroom fit-out. We've seen these run from around £10,000 to well over £100,000 depending on what you want, so putting a number on it here would be more misleading than useful. It just flags that cost as separate.
Still not a quote
The calculator gives a range, not a figure, because that's the honest way to answer a question that genuinely varies job to job. Ground conditions, drainage, access and the exact specification all move the final price, and the only way to account for those properly is a site visit. We say this on the calculator itself, and we'll say it here too: treat it as a starting point for a conversation, not a number to hold us to.
Try it, then talk to us
Use the calculator to get a rough range for your own project. Once you're done, there's an option to send us a summary of what you selected directly, so if it looks like something worth pursuing, you can go straight from a rough figure to booking a proper site visit. See our guide to what actually drives an extension's cost for more on the factors behind the numbers.
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About the author
Written by JLS Building & Property Development Ltd, a family-run building contractor covering Bedfordshire, Cambridgeshire, Hertfordshire and Buckinghamshire. FMB member and SafeContractor approved. More about us. This guide is general information, not professional or legal advice for your specific project.