The Honest Extension Cost Guide
Everyone wants a number. The honest answer is a range — but a useful one. Here's what extensions really cost in the UK, including the bits people forget, so you can budget with confidence.
The per-square-metre starting point
As a rule of thumb, build costs run roughly £2,000–£3,000 per m². A 20m² single-storey extension therefore costs around £40,000–£60,000 to build — before kitchen units, furniture and finishes.
Two-storey extensions are often better value per m², because you share the cost of foundations and roof across more floor space.
The costs people forget
Design and planning fees, structural calculations, building control, party wall agreements, VAT, and a contingency of 10–20%. Leave these out and your budget will blow up mid-build.
Our design fee is always fixed and agreed before any work starts, so at least one big number never moves.
Where to spend and where to save
Spend on the things you touch every day and the things that are hard to change later — glazing, layout, worktops. Save on what's easy to upgrade or what nobody notices.
The honest takeaways
- ✓Budget roughly £2,000–£3,000 per m² for the build.
- ✓Always keep a 10–20% contingency — something always comes up.
- ✓Get the design right first; changes on site are the biggest cost killer.
Questions people ask
Does your fee come out of the build budget?
No — our design fee is separate and fixed. Build costs are paid to your builder. We help you budget for both honestly.
Why do quotes vary so much?
Spec, ground conditions and builder quality all move the number. The cheapest quote is rarely the cheapest project once extras and rework are counted.
Not sure whether to build, wait or move?
That's exactly what the call is for. We'll be straight with you — no pressure, no jargon, no nasty surprises.