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 — see our side return extension cost breakdown. 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
Single vs double-storey economics — and where homeowners overspend
Going up is cheaper per square metre than going out. Because a double-storey extension shares the same foundations and roof across two floors, the £/m² rate typically falls 10–20% below a single-storey of the same footprint. But the headline bill is bigger: a double-storey usually costs 50–70% more than an equivalent single-storey — not double — while giving you twice the internal floor area. If the plot suits building up and you need the rooms, it is often the stronger value per pound; read our single vs double-storey cost comparison and the full extension cost by size breakdown to sense-check the total against your footprint.
Where the money leaks. Ranked by how often they blow a budget: (1) kitchen and worktop spec — a quartz-and-island upgrade quietly adds £8,000–£15,000; (2) glazing creep — swapping standard windows for a bi-fold-plus-lantern wall late in the design; (3) ground conditions nobody surveyed — clay, trees or made-up ground found only after digging; and (4) scope changes mid-build — moving a wall or adding underfloor heating once the slab is down. The first two are choices you control up front; the last two are why a proper site survey and a fixed, itemised quote matter. Confirm permitted-development limits early too — the Planning Portal extensions guidance sets the 3m/4m single-storey rear limits (rising to 6m/8m under prior approval), and our do I need planning permission for an extension guide covers the exceptions that push a project into full planning.
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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