How Much Does a House Extension Cost UK 2025?
The honest answer: it depends on where you live, what you are building and how good a finish you want. But here are realistic figures based on actual projects, not wishful thinking.
Cost Per Square Metre (2025)
| Extension Type | Budget | Mid-Range | High-End |
|---|---|---|---|
| Single storey rear | £1,200/sqm | £1,600/sqm | £2,000+/sqm |
| Double storey | £1,000/sqm | £1,400/sqm | £1,800+/sqm |
| Loft conversion | £1,000/sqm | £1,250/sqm | £1,500+/sqm |
| Basement | £2,000/sqm | £2,800/sqm | £3,500+/sqm |
Prices include VAT. London and the South East typically add 15-25% to these figures. Scotland, Wales and the North are usually at the lower end.
What Is Included in These Costs
- - Foundations, walls, roof structure and weatherproofing
- - Windows and external doors
- - Electrics and plumbing (first fix and second fix)
- - Plastering, flooring and basic decoration
- - Building Regulations inspections
Hidden Costs You Need to Budget For
Regional Variations
Labour costs vary significantly across the UK. A builder in London might charge £200-250 per day, while the same skill level in the North West charges £150-180. Materials cost roughly the same everywhere, but labour is typically 40-50% of the total build cost, so location has a big impact.
Worked Examples
Example 1: 4x5m Single Storey Rear (20sqm) — Midlands
Example 2: Double Storey Side and Rear (40sqm) — South East
Example 3: Dormer Loft Conversion (25sqm) — North West
Tip: Always Add a 10-15% Contingency
Every building project hits unexpected problems: ground conditions, hidden asbestos, structural issues only visible once work starts. Budget for at least 10% contingency on top of every quote. If you do not use it, consider it a bonus.
Tip: Does It Add Value?
A well-designed extension typically adds more value than it costs, but not always. A single-storey rear extension creating an open-plan kitchen-diner adds the most value per pound spent. Basement conversions rarely recoup their cost outside of London. Get a local estate agent to value your property before and after to check the numbers stack up.
Prices based on 2025 market rates. Costs increase roughly 3-5% per year. Always get at least three written quotes from local builders and check references.
← Back to all guides