Ask three builders what an extension costs and you will get three answers, usually vague ones. So here is a straight answer first: for a well-specified single storey extension in Surrey in 2026, budget £2,400 to £3,200 per square metre of new floor area, before VAT and professional fees. A typical 25 square metre kitchen extension, once you include glazing, the kitchen itself and finishes, usually lands somewhere between £75,000 and £110,000.
No two projects price the same, though. What follows is how that number is actually built up, so you can see where the money goes and which decisions move it most.
The headline ranges
- Single storey extension: £2,400 to £3,200 per square metre, depending on specification
- Double storey extension: a lower rate per square metre, since one roof and one set of foundations serve two floors, but a higher total
- Wraparound or heavily glazed extensions: £3,200 per square metre and upwards
- Basement work: a different league entirely, rarely below £4,000 per square metre
These figures assume a competent design, sensible ground conditions and a complete specification. They exclude VAT, professional fees and the kitchen or sanitaryware, which vary too much between projects to generalise.
What moves the number most
Structure and glazing are the two big levers. A modest opening with a standard steel beam is routine. Removing the whole rear wall, or carrying two storeys above a large span, needs heavier steel, more temporary works and more engineering time. Glazing behaves the same way: a well-made set of bifold doors is a known cost, while structural glass, gable glazing and roof lanterns climb quickly.
Below ground matters just as much, and it is the part nobody sees. Much of Surrey sits on clay, which often means deeper foundations, and nearby trees or drains can push depths further. This is why a proper survey before pricing is worth far more than a hopeful estimate after.
The costs people forget
- VAT at 20 percent, which most builder quotes exclude
- Professional fees, typically 10 to 15 percent across architecture, structural engineering and building control
- Party wall agreements where you build near a boundary
- A contingency of around 10 percent, which a well-run project holds and rarely spends
How to keep control of it
Fix the design before you price it. Quotes are only comparable when they are priced against the same complete specification, and most budget overruns start life as a gap in the drawings rather than a builder's error. One accountable team taking the project from first drawing to handover removes the gaps where costs usually hide.
If you are weighing up an extension and want a realistic figure rather than an optimistic one, we are happy to talk it through.
