If a client asks "how much per square metre for an extension?" — the honest answer is: it depends. But that's not useful. Here's a practical guide to the real cost ranges UK builders are quoting in 2026, and the factors that push prices up or down.
Current Cost Per m² by Extension Type (UK, 2026)
| Extension Type | Budget (£/m²) | Mid-range (£/m²) | Premium (£/m²) |
|---|---|---|---|
| Single-storey rear extension | £1,600–£1,900 | £2,000–£2,500 | £2,600–£3,500+ |
| Two-storey extension | £1,400–£1,700 | £1,800–£2,200 | £2,400–£3,000+ |
| Side-return extension | £2,000–£2,400 | £2,500–£3,000 | £3,200–£4,500+ |
| Wrap-around extension | £2,200–£2,600 | £2,800–£3,500 | £3,800–£5,000+ |
| Basement conversion/extension | £3,000–£4,000 | £4,200–£5,500 | £6,000–£8,000+ |
Note: Two-storey extensions have a lower cost per m² because foundations, drainage, and roof are shared across two floors.
What's Included in These Figures?
The mid-range cost per m² figures above typically include:
- Groundworks and foundations
- Structural walls (brick or block)
- Roof structure and covering (flat EPDM or pitched tiles)
- Windows and external doors (uPVC)
- First fix electrics and plumbing
- Insulation to Building Regs standard
- Plasterboard and skim
- Second fix electrics and plumbing
- Basic floor screed or timber floor
They typically exclude:
- Internal decoration
- Kitchen or bathroom fitting
- Flooring finishes
- Bifold or sliding doors (add £3,000–£8,000)
- Underfloor heating (add £80–£150/m²)
- Planning fees and structural engineer
- VAT (if your business is VAT registered)
Regional Price Variations
Labour rates vary significantly across the UK. As a rough guide, apply these multipliers to the mid-range figures above:
- London: +30–40%
- South East: +15–25%
- South West: +5–10%
- Midlands: Base rate
- North of England: -5–10%
- Scotland / Wales: -5–15%
What Pushes the Price Up?
The biggest drivers of cost above the base rate:
- Bi-fold or aluminium sliding doors — These add £4,000–£10,000 and are the single most common upgrade on modern extensions.
- Flat roof vs pitched — A well-spec'd flat roof (EPDM or GRP) adds complexity. Lantern lights add another £2,000–£4,000 each.
- Underpinning existing foundations — If the existing structure needs strengthening, costs escalate rapidly. Always flag this as a provisional sum.
- Heritage properties — Listed buildings or conservation area homes require approved materials and specialist labour. Budget 20–30% extra.
- Tricky access — London side-returns in terraced rows, or rear extensions with no garden access, mean all materials come through the house. Price accordingly.
The Minimum Viable Extension
For homeowners asking "what's the minimum I can spend on an extension?" — the realistic floor for a 20m² single-storey rear extension with basic spec in the Midlands is around £35,000–£40,000. Below that, you're either cutting corners on insulation and structure (a risk), or using a builder who will struggle to deliver at that price.
How to Build an Accurate Extension Quote
Cost per m² is a starting point, not a quote. To protect your margin on extension work:
- Price each trade/section separately — don't lump everything into a single m² rate
- Get live material prices at the point of quoting — timber, blocks, and roof materials fluctuate
- Add a 10–15% contingency for groundworks specifically
- Use a variation clause — any change to the agreed spec is a chargeable variation
- Track actual costs weekly against your estimate as the job runs
QuoteBuild includes UK trade price libraries so you can build itemised extension quotes in minutes and track your live margin as costs come in — no spreadsheets required.
If extensions are part of your regular workload, see how live tracking works or check the flat pricing.
If you mainly quote builder projects, see the builder page or browse all trade pages.