House extension cost calculator (UK)

What the number actually includes

The build cost above covers the whole shell and fit-out: foundations, drainage connections, brickwork or render, roof, doors and windows, plastering, first and second fix electrics and plumbing, decoration and a standard floor finish. Professional fees cover drawings, structural calculations, building control and — if you need it — the planning application. The contingency is money you should genuinely expect to spend.

The five things that move the price most

1. Ground conditions

Nobody knows what the foundations will cost until the trench is open. Clay soil, a mature tree within influencing distance, made ground or an old cellar can turn a £4,000 foundation into a £20,000 one. Any builder who prices foundations to a fixed sum without a trial hole is either very confident or about to hand you a variation.

2. Drains

If a public sewer runs under or within three metres of the extension you will need a build-over agreement from the water authority, and possibly a diversion or a new manhole. It is rarely more than £4,000 but it is almost never in the first quote.

3. Glazing

A three-panel bi-fold and a slim-frame aluminium slider look similar in a drawing and differ by £12,000 in reality. Glazing is the easiest place to save money without the room feeling cheaper — and the easiest place to lose control of a budget.

4. What the extension connects to

Extensions rarely stop at the new walls. Removing the old rear wall needs a steel beam and padstones. Adding 20m² of underfloor heating may need a bigger boiler. The old kitchen becomes a utility. Budget for the works either side of the join.

5. Access

A terraced house where every barrow of muck and every pack of blocks goes through the hallway costs meaningfully more than a detached with a driveway. It is labour, not materials — but it is real, and it is why the same drawing prices differently street to street.

Where extension budgets usually go wrong

• Comparing quotes that cover different work. Put every quote onto the same line-by-line schedule before you compare totals. It is the single most valuable hour you will spend.
• Forgetting the finishes. Flooring, blinds, lighting, garden reinstatement and decorating the rooms you disturbed will add £5,000–£15,000 that no builder quoted.
• Paying too much up front. Stage payments should follow completed work, not the calendar. A large deposit before anything arrives on site is a risk you do not need to take.
• Not costing the disruption. Six months without a kitchen has a price, even if it is not on an invoice.

Will you get the money back when you sell?

Sometimes. A well-built extension that adds a bedroom or opens up the ground floor typically returns most of its cost in a rising market, and adding floor area is one of the few improvements valuers reliably reward. But every street has a ceiling price, and spending £90,000 on a house four doors from the top of the road rarely comes back. If resale is your main reason for building, get a local agent's view on the ceiling before you commit.