How Much Does a House Extension Cost in 2026?

Q: How long does an extension take to build?

Most single-storey extensions take 3–4 months on site; larger or double-storey projects run 4–6 months, plus design and approval time beforehand.

What a single or double-storey extension really costs in the UK in 2026 — per square metre, by type, and the hidden costs to budget for.

Typical UK extension costs in 2026

Most house extensions cost between £20,000 and £70,000 in 2026, or roughly £1,800–£3,000 per square metre. A single-storey extension typically runs £20,000–£45,000; a double-storey adds far more floor area for a smaller jump, around £45,000–£90,000; and a large kitchen-diner or wrap-around at the top end can exceed £100,000.

What drives the price

Size and finish matter most, then groundworks, structural steel, glazing (bifolds and rooflights are pricey), and whether you’re moving a kitchen or bathroom into the new space. London and the South East sit well above the national average.

Don’t forget the extras

Beyond the build, budget for design and structural drawings, planning or building-regs fees, party-wall agreements with neighbours, and VAT. A sensible rule is to add 10–15% contingency on top of the quote.

Single vs double storey — which is better value?

A double-storey extension usually costs only around 50% more than a single-storey but can double the added floor space, so the cost per square metre is often lower. If your plot and budget allow, it’s frequently the better long-term value.

Get a real figure for your home

Costs vary hugely by design and region. Share your plans and we’ll match you with vetted local extension builders for accurate, no-obligation quotes.

Frequently asked questions

How long does an extension take to build?

Most single-storey extensions take 3–4 months on site; larger or double-storey projects run 4–6 months, plus design and approval time beforehand.

Does an extension add value?

A well-designed extension usually adds value, and a kitchen-diner or extra bedroom often returns much of its cost — though you can hit a local price ceiling in some areas. Build quality and design matter more than size.

Do I need an architect?

For anything structural you’ll need drawings and often a structural engineer. An architect isn’t legally required, but professional design usually pays for itself on larger or more complex extensions.