Extension planning guide
Extension Cost per m² UK
How much does a house extension cost per m2 in the UK? See current building rates for single storey, double storey, brick, timber frame and SIPs construction.
Typical budget range
£30,000–£120,000+
Main cost drivers
Size, specification, glazing, foundations
Best use
Feasibility before design spend
Extension example
The kind of rear extension result people picture — but the real budget is driven by structure, spec and ground conditions.
Extension guide snapshot
Per-m² figures help, but hidden build costs still matter
Foundations, steelwork, glazing packages and fit-out choices can swing the project far beyond the headline rate.
Remember
A crisp-looking extension budget can still be wrong if it ignores enabling works, drainage and finishing items.
Good next step
Use the extension calculator after reading to compare realistic size and spec combinations.
Extension Cost per m² by Construction Type
| Construction Type | Cost per m² (Low) | Cost per m² (High) | Typical Spec |
|---|---|---|---|
| Timber Frame | £1,600 | £2,400 | Builder finishes |
| Brick & Block (standard) | £1,800 | £2,800 | Builder finishes |
| Single Storey (brick & block) | |||
| Basic specification | £1,800 | £2,400 | No kitchen fit-out |
| Mid specification | £2,400 | £3,000 | Mid kitchen, decent finishes |
| High specification | £3,000 | £4,000 | Premium finishes throughout |
| Double Storey | |||
| Cost per m² | £2,000 | £3,200 | Floors share foundations |
These costs are for the build itself — excluding architect fees (typically 5–10% of build cost), planning fees, and structural engineer fees.
Why Larger Extensions Cost Less Per m²
Economies of scale apply strongly to extensions. A 15 m² single-storey extension will cost significantly more per m² than a 60 m² one. The fixed costs — foundations, builder's preliminaries, plant hire, skip — are spread across more square metres.
This is why it's often worth planning a larger extension than you currently need — the marginal cost of extra square metres is much lower than the average cost.
Regional Differences
London is consistently 35–45% above the national average. A single-storey extension that costs £2,500/m² in the Midlands will cost £3,375–£3,625/m² in London. Scotland and the North East are typically 5–15% below the national average.
Specification Impact on Cost
- Basic spec — standard insulation, vinyl flooring, basic kitchen cabinets, standard sanitaryware
- Mid spec — better insulation (SAP 10 compliant), oak flooring or good quality tile, mid-range kitchen, quality sanitaryware
- High spec — premium insulation, underfloor heating, full acoustic separation, high-end kitchen (£15,000+), designer bathroom
- Luxury spec — smart home integration, polished concrete or marble floors, bespoke kitchen (£30,000+), premium bathroom brands
What Else to Add to Your Budget
- Architect fees — 5–10% of build cost
- Structural engineer — £1,000–£3,000
- Planning application fees — £206–£500 depending on size
- Building regulations inspection — typically £500–£1,500
- Party wall agreements (if semi or terrace) — £700–£2,000
- Utility connections / diversions — £500–£3,000
- Landscaping to match new floor level — £2,000–£8,000
Calculate your extension cost
Get an indicative budget including size, specification and region.
Open Extension Cost Calculator →