Compare Extension Costs: What Affects Price in 2026 (UK)
Rear single-storey extensions cost £1,800–£2,800/m² in 2026. Side return infill (terraced and semi-detached) costs £2,000–£3,200/m². Wraparound (rear + side combined) costs £2,200–£3,500/m². Double-storey adds £700–£1,300/m² over single-storey. But the headline rate hides the four biggest swing factors: glazing area, kitchen spec, ground conditions, and the structural opening between extension and existing house.
Rear vs side return vs wraparound vs double-storey — 2026 UK
Rear single-storey — the most common UK extension. Costs £1,800–£2,800/m². Typical 4×6m (24m²) extension: £43,000–£67,000 shell plus kitchen and finishes. Usually permitted development if within size limits.
Side return — infills the alley at the side of a terraced or semi house. £2,000–£3,200/m². Smaller footprint (typically 8–14m²) but proportionally more expensive due to party-wall agreements, neighbour access and tricky existing fabric. Adds £15,000–£45,000 to a Victorian terrace and 8–12% to value.
Wraparound — rear + side combined. £2,200–£3,500/m². Typically 25–40m² total. £55,000–£140,000+. The favourite for end-of-terrace and semi-detached open-plan kitchen-diners.
Double-storey — ground floor extension PLUS first floor above. Add £700–£1,300/m² over single-storey because foundations and roof are amortised across two floors. Typical 24m² ground + 24m² first = 48m² total at £2,500–£3,500/m² = £120,000–£168,000. Often requires planning permission.
What actually drives the price (beyond footprint)
1. Glazing area
The biggest variable. Standard double-glazed window: £400–£700 each. Aluminium bi-fold doors: £900–£1,500/lin m. Sliding patio doors: £1,100–£1,800/lin m. Roof lanterns: £1,400–£3,500 each. A glazing-heavy extension (e.g. 5m bi-fold + 3 roof lights) easily adds £8,000–£15,000 over a standard window-only spec.
2. Kitchen and bathroom spec
Headline £/m² rates EXCLUDE kitchens. Kitchen units & worktops: £6,500–£20,000+. Bathrooms: £5,500–£12,000 each. These are the easiest places to over-spec — a quartz worktop alone is £200–£500/m² against laminate at £30–£80/m².
3. Ground conditions and foundations
Clay shrinkage zones (London, South-East): trench fill foundations to 1.0–2.0m deep, adds £1,500–£5,000+ over a normal 600–900mm trench. Nearby trees: 1.5× tree height clearance or root-deflecting piles (£6,000–£14,000). Made-up ground (former industrial): piled raft (£10,000–£25,000+). Get the structural engineer to flag ground conditions before quoting.
4. Structural opening to existing house
The big steel between extension and existing kitchen sits at £500–£1,500 for the structural engineer fee plus £600–£3,000 for the steel itself plus padstone, brick "jumping", plastering and finishing — total £2,500–£6,500. Underestimated in 40% of self-managed extensions.
Extension cost by UK region (2026)
Single-storey rear extension £/m²:
- London: £2,300–£3,500/m² (clay shrinkage zone — deep foundations)
- South East: £2,100–£3,200/m²
- South West, East, Midlands: £1,800–£2,800/m²
- North West, Yorkshire, North East: £1,650–£2,500/m²
- Scotland & Wales: £1,600–£2,400/m²
How to get a fair extension quote
1. Pick type and footprint
Use the per-m² bands above as a sense-check.
2. Cost glazing separately
Schedule every window, door and roof light. Get the builder to itemise.
3. Use a provisional sum for kitchen & bathroom
Keep these out of the build cost and decide spec independently.
4. Confirm structural engineer and steel
Make sure these are in the quote.
5. Compare three written quotes
Use BestBuilders to match with three vetted local extension builders.
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