House Extension Cost Calculator UK
Get a realistic 2026 UK extension cost estimate in under a minute. Choose type, size, finish and region — we show low/mid/high figures with 10% fees and a 15% contingency built in, based on builder pricing across 519 UK towns cross-referenced with RICS BCIS. No email required.
How Much Will Your Extension Cost?
5 steps. Base £/m² by type × your floor area × finish multiplier × regional rate, plus ~10% professional fees and a 15% contingency buffer.
The Maths Behind Your Estimate
No black-box magic. We start with the 2026 UK base £/m² for your extension type, apply your finish and regional multipliers, then add professional fees and a contingency buffer.
Base Rates (£/m², by type)
- Single-storey rear — £2,000/m² (most popular)
- Single-storey side return — £2,100/m²
- Double-storey — £2,200/m² (footprint doubled across two storeys)
- Wraparound — £2,300/m² · Porch — £1,700/m²
- Loft conversion — £1,500/m²
Finish: Basic ×0.85 · Standard ×1.00 · Premium ×1.35 · Luxury ×1.70.
Fees, Contingency & Region
- +10% fees — planning, building regs, architect, structural engineer
- +15% contingency — recommended buffer for surprises
- London +35% · South East +15% · Midlands ±6–8%
- North East −15% · Northern Ireland −18% (lowest)
- Low/high band: mid ×0.88 to ×1.15 (matches real quote spread).
Indicative only, based on 2026 UK builder pricing across 519 towns + RICS BCIS. A real price needs a site survey.
Typical Extension Costs (2026, All-In)
base £/m² × size × finish + 10% pro fees + 15% contingency, Midlands baseline.
| Extension type & spec | Mid estimate | Cost range (all-in) |
|---|---|---|
| Single-storey rear, 20m², standard finish | £50,600 | £44,528–£58,190 |
| Side-return, 15m², standard finish | £39,848 | £35,066–£45,825 |
| Single-storey rear, 30m², premium finish | £102,465 | £90,169–£117,835 |
| Double-storey, 40m², standard finish | £111,320 | £97,962–£128,018 |
| Wraparound, 35m², premium finish | £137,474 | £120,977–£158,095 |
These figures power the calculator above — enter your own spec for a tailored estimate. For the full breakdown, read our complete cost guide.
Extension Costs at a Glance: 2026 Visual Guide
Where you build moves the budget more than any specification choice. The chart plots the mid-points of the 2026 regional ranges used across this page: a single-storey extension that prices at about £1,925/m² in the North East climbs to £3,125/m² in Greater London and £3,700/m² in inner and prime postcodes — roughly 90% above the cheapest English regions, before you have chosen a single finish.
Take the £50,600 mid estimate above for a 20m², standard-finish single-storey rear extension and split it the way a quantity surveyor prices a tender. The structural stages — groundworks, shell and roof — absorb around 47p in every pound, while professional fees (8%) and the 15% contingency built into every figure on this page take a fifth of the total before you spend anything on the parts you actually see.
On site, a straightforward single-storey extension runs to a rhythm every good builder knows. The programme below shows a typical 12-week build — the fast end of the 12–16 week range our calculator quotes. The milestone that matters is watertight at the end of week seven: once the roof is on and the glazing is in, weather can no longer stall the job and the internal trades can be booked with confidence.