How Much Does a House Extension Cost in 2026?

A transparent breakdown of extension costs in the UK — covering single-storey, double-storey, and specialist builds with realistic 2026 pricing. ## Extension Costs by Type | Extension Type | Cost per m² | 15m² | 25m² | 40m² | |---|---|---|---|---| | Single-storey rear | £1,800–£2,800 | £27,000–£42,000 | £45,000–£70,000 | £72,000–£112,000 | | Double-storey rear | £1,400–£2,200 | — | £70,000–£110,000 | £112,000–£176,000 | | Side return infill | £2,000–£3,000 | £30,000–£45,000 | — | — | | Wrap-around | £1,800–£2,500 | — | £45,000–£62,500 | £72,000–£100,000 | | Loft conversion (dormer) | £1,200–£1,800 | £18,000–£27,000 | £30,000–£45,000 | — | These figures include foundations, structure, roof, electrics, plumbing, plastering, and basic decoration.

Kitchen and bathroom fit-out is additional. ## Cost Breakdown: Where the Money Goes For a typical 20m² single-storey rear extension at £2,200/m² (£44,000 total): | Element | Approximate % | Cost | |---|---|---| | Foundations and groundwork | 15% | £6,600 | | Structural walls and steel | 20% | £8,800 | | Roof structure and covering | 12% | £5,280 | | Windows and doors | 10% | £4,400 | | Electrics | 8% | £3,520 | | Plumbing and heating | 8% | £3,520 | | Plastering and decoration | 10% | £4,400 | | Flooring | 5% | £2,200 | | Professional fees | 7% | £3,080 | | Contingency | 5% | £2,200 | ## What Affects the Price?

**Ground conditions** — clay soils, high water tables, or proximity to trees can require deeper or piled foundations, adding £3,000–£10,000. **Specification level** — basic (painted plaster, laminate floors) vs premium (underfloor heating, bi-folds, engineered oak) can double the per-m² cost. **Structural complexity** — large steels for open-plan layouts, party wall agreements, or building over drains add cost and time.

**Professional fees** — architect, structural engineer, and Building Control fees typically total 7–12% of build cost. **Location** — London and the South East command 15–30% premiums over national averages. Hampshire sits slightly above the national average. ## Professional Fees to Budget For | Service | Typical Cost | |---|---| | Architect / designer | £2,000–£5,000 | | Structural engineer | £800–£2,000 | | Planning application | £206 (householder, 2026) | | Building Control | £400–£1,200 | | Party Wall surveyor | £700–£1,500 per neighbour | ## What a Quote Should Include - Detailed specification of all works - Foundation design assumptions (with caveats for ground conditions) - All structural steelwork - Full electrical and plumbing first and second fix - Plastering and decoration - Building Control fees - Waste removal - Timeline with key milestones - Payment schedule linked to stages, not calendar dates --- ### What to do next If you are planning a house extension, EvoHome Improvements can help you organise the details, compare options and, where appropriate, connect the project with suitable independent installers or specialists.

EvoHome does not carry out the works directly.…