How much does a house extension cost in the UK?
A 2026 guide to single and double-storey extension costs per square metre, plus fees.
Extensions are usually priced per square metre for the construction, with design, planning and VAT on top. Here are typical 2026 rates to help you budget before you speak to builders.
Typical extension costs
| Type | Typical cost (2026 guide) |
|---|---|
| Single-storey (UK) | £2,000–£3,500 / m² |
| Single-storey (London) | £2,500–£5,000 / m² |
| Double-storey | add ~50–60% to single-storey total (lower per m²) |
| Design & planning fees | usually extra |
| VAT | usually extra |
These are typical guide prices for 2026 and will vary by job, property and location. Always get written quotes for your own project before deciding.
What affects the cost?
- The size and specification of the extension
- Groundworks, foundations and structural elements
- The quality of finishes and fittings
- Access to the site and materials storage
- Design, structural engineer and building-control fees
- London labour rates (10–25% above the national average)
Extension costs in London
In London, single-storey extensions commonly run £2,500–£5,000 per square metre, with central boroughs 20–40% higher than outer London. Remember quotes are usually for construction only — add VAT and professional fees.
How to get a fair price
Get detailed, like-for-like quotes from more than one builder, use a written contract with a staged payment plan, and keep a retention until snagging is complete. Check FMB or TrustMark membership and that building regulations and any planning permission are in place.
Frequently asked questions
How much does a single-storey extension cost?
As a guide, £2,000–£3,500 per square metre across the UK, or £2,500–£5,000 per square metre in London, for construction only. A typical 20 m² extension therefore often falls between £40,000 and £100,000 depending on location and finish.
Is a double-storey extension cheaper per square metre?
Usually yes. Because the foundations and roof are shared across two floors, a double-storey extension often costs less per square metre than a single-storey one — though the total is higher.
Do these prices include VAT and fees?
No. Extension quotes are typically for construction only. Budget separately for VAT, architect or designer fees, structural engineer costs and building-control charges.
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