Cost calculator

House extension cost calculator

A house extension price is driven far more by ground conditions, structural openings and finish level than by floor area alone. Most quotations you receive will be a shell-and-core price with a provisional sum bolted on for kitchens, bathrooms and flooring — which is exactly where two quotes for the same job stop being comparable.

Extension floor area

Internal floor area of the new space, all storeys added together.

25 m²

Region

Broad trade availability keeps day rates near the national average.

Estimated range, ex VAT

£45,500 – £78,000

Labour (typical): £34,000
Materials (typical): £28,000

Guidance only. A real quotation depends on site conditions, specification and access — which is exactly what an independent review of your own quote will tell you.

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What affects the cost

Foundation depth

Trees, clay soil, drains or a shared boundary can turn a 900mm trench into a 2.5m dig or a piled solution. This is the single biggest overrun risk on extensions.

Structural openings

Every steel beam needs design, supply, installation and fire protection. A wide open-plan opening into the existing house can add £3,000–£9,000.

Glazing specification

Bifolds, sliders and rooflights range from around £900/m² to over £2,500/m² fitted. Many quotes carry a low allowance you will later exceed.

Drain diversions

Building over or near a public sewer needs a build-over agreement and often a manhole relocation.

Finish level

Plaster, joinery, tiling and electrics vary enormously. A 'developer' finish and a bespoke finish can differ by 40% on the same shell.

Regional pricing differences

Materials cost roughly the same everywhere in the UK; labour does not. Use the region selector above to apply a local index — London and the South East typically run 15–30% above the national average, while the North East, Northern Ireland and Yorkshire sit below it.

Labour vs materials

Roughly 55% of this type of work is labour and 45% materials. Groundworks, bricklaying and carpentry dominate labour. Steel, glazing and roof coverings dominate materials, and those have moved sharply in the last three years.

Common hidden costs

Building control fees, structural engineer's calculations and party wall surveyor fees (often £1,000–£3,500 combined).
Making good the existing house where the extension ties in — plastering, flooring levels and redecoration.
Temporary works: scaffold hire beyond the quoted weeks, and skips once the muck-away volume is known.
Utility alterations — moving a boiler flue, gas meter or electricity supply is charged by the utility, not the builder.

How to save money

Keep the roof line simple. A hipped or gable roof with standard trusses is far cheaper than a flat roof with structural glazing.
Avoid moving soil pipes and the kitchen's existing service runs where the layout allows it.
Fix your kitchen, bathroom and floor finishes before signing so provisional sums become real numbers.
Get three quotes priced against the same drawings and specification — not against a verbal brief.

Frequently asked questions

How much does a 4m x 5m extension cost in the UK?

A 20m² single storey extension typically lands between £38,000 and £62,000 ex VAT nationally, before kitchen and flooring. London adds roughly 25–30%.

Is a double storey extension cheaper per square metre?

Yes. You reuse the same foundations and roof, so the second storey usually costs 25–40% less per m² than the ground floor.

Do I pay VAT on an extension?

Extensions to an existing dwelling are normally standard-rated at 20%. Some conversions and properties empty for 2+ years qualify for 5% — ask your contractor to state the VAT position in writing.

How long does an extension take?

Allow 10–14 weeks for a straightforward single storey, 16–24 weeks for a double storey or anything with significant structural work.

Independent

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Evidence-linked

Every finding points back to the exact page and wording in your quote.

Private

Your quotation is used to produce your report and nothing else.