What does a house extension really cost in 2026?

A straightforward guide for UK homeowners — no jargon, just numbers

Buildcostr.ai · May 2026 · 5 min read

The short answer

A typical UK house extension costs between £1,500 and £3,000 per square metre for the build alone — depending on specification, location and complexity. A 30m² single-storey extension will typically cost £45,000 to £90,000 in materials and labour before professional fees and VAT.

What drives the cost

Size. Larger extensions cost more in total but often less per square metre, as fixed costs like scaffolding and site setup are spread across more floor area. Doubling the footprint rarely doubles the cost.

Specification. Finishes make an enormous difference. A basic-spec extension might land near £1,500/m², while premium glazing, bespoke joinery and high-end kitchens or bathrooms can push you well past £3,000/m².

Location. Labour rates vary significantly across the UK. The same extension built in London can cost 20–40% more than one built in the Midlands or the North due to higher labour and overhead costs.

Complexity. Steel beams, removing structural walls, tricky access, sloping sites and extensive groundworks all add cost. The more structural intervention a design requires, the higher the build figure climbs.

Regional cost differences

London and the South East carry a clear premium — expect to pay toward the upper end of every range and often beyond. A 30m² extension that costs around £55,000 in a regional town can comfortably exceed £75,000 in London for the same specification, driven almost entirely by labour and overhead rates rather than materials.

What is not in the build cost

• Architect and design fees
• Planning application fees
• Building regulations fees and inspections
• Party wall agreements
• Furniture, appliances and loose fittings

How to get an accurate estimate

The only way to know what your specific project should cost is a proper, itemised cost plan based on your actual drawings. Buildcostr.ai produces exactly that — a professional, QS-methodology cost plan in 30 minutes — so you can approach builders knowing what a fair quote looks like. You can see a full sample cost plan before you buy, or order yours now.

Pricing the work yourself? Read why builders lose money on residential jobs and how to avoid the same mistakes.