Extension Cost per m² UK
Extension costs per m² in the UK range from around £1,800 to £4,000+ depending on construction type, specification and location. Here's the full breakdown.
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
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