Extension Cost Calculator — UK Build Cost Estimator

In 2026, a single storey extension typically costs between £2,000 and £3,000 per m² (excluding VAT) depending on location, specification and ground conditions, per 2026 cost guides from Checkatrade, MyJobQuote and Less.co.uk. London and the South East sit at the upper end (often £2,650–£4,200/m²), while northern England and Wales are generally 10–15% lower. These figures include foundations, structure, roof, electrics, plumbing and finishing but exclude professional fees and VAT. The calculator uses an indicative £2,150/m² single-storey baseline — treat it as a budget estimate, not a quote.

A two storey extension shares the same foundation and roof as a single storey, so those fixed costs are spread over double the floor area. The upper floor adds walls, a floor structure and staircase but avoids duplicating the most expensive elements. Typical two storey costs run roughly £1,700–£2,500/m² compared to £2,000–£3,000/m² for single storey (2026 guides).

The breakdown covers foundations (concrete, excavation, reinforcement), structure (blockwork, brickwork, cavity insulation, DPC), roof (tiles or flat roof membrane, insulation, fascia), windows and doors, first and second fix electrics, plumbing and heating, and internal finishing (plastering, flooring, decoration). Professional fees, planning and Building Regulations fees are not included.

A contingency of 10–15% is standard for domestic extension projects. This covers unforeseen issues such as poor ground conditions requiring deeper foundations, discovering asbestos, hidden drainage runs, or changes to the specification during the build. For older properties or complex sites, 15–20% is prudent.

Yes, significantly. London and the South East can be 20–30% more expensive than the national average due to higher labour rates and material delivery costs. Northern England, Wales and Scotland tend to be 5–15% below the national average. This calculator applies regional multipliers to give location-adjusted estimates.

Yes. An extension to an existing home is standard-rated at 20% VAT (HMRC VAT Notice 708), so a registered builder charges 20% on labour and materials. The calculator's build-cost figures are shown ex-VAT, with a separate 20% VAT line and an inc-VAT total. The exceptions are a brand-new self-contained dwelling (zero-rated) and renovations to a home that has been empty for two years or more (a reduced 5% rate may apply). Separately, the installation of specific energy-saving materials — insulation, heat pumps, solar panels and heating controls, but not underfloor heating itself — is zero-rated until 31 March 2027 under VAT Notice 708/6 (reverting to 5% from 1 April 2027); wet underfloor heating only gets that rate if it forms part of a qualifying installation such as a heat pump.

Wet (water-based) underfloor heating in a new screed floor — the cheapest scenario, because the floor is being built anyway — runs about £70–£120 per m² supply and fit (2026 guides from MyJobQuote and Checkatrade). The calculator uses ~£90/m² for the floor area you enter. Retrofit over an existing floor is dearer at roughly £100–£190/m². Electric (dry) mat systems are cheaper to install but more expensive to run.