Advanced options
Per-m2 rates are shell + standard finish. Kitchens and bathrooms inside the extension add their own budgets (a kitchen easily GBP 10-25k). The 15% covers architect/structural engineer/building control and the contingency every build uses. Double storeys are the bargain per m2: the roof and foundations are already paid for. Rough guide only: three itemised quotes beat any calculator.
What The Extension Cost Calculator Works Out
- Build cost: floor area multiplied by the per-m2 rate the trade quotes for shell and standard finish: around GBP 2,000 to GBP 2,500 for basic single-storey work, GBP 2,500 to GBP 3,000 for the good standard most families build, and GBP 3,000 to GBP 4,000 where London labour or high specification apply. A 20 m2 kitchen-diner at mid-spec is GBP 54,000 before the add-ons.
- Double storey: the second floor rides on foundations and roof already paid for, so it costs roughly 60% more rather than double, which makes it the bargain per square metre.
- Realistic total: build cost plus 20% VAT and a further 15% for the architect, structural engineer, building control and the contingency every project spends. Building Regulations always apply even when planning does not.
- On top of that: fit-outs, party wall costs and planning fees that per-m2 rates exclude, listed as a reminder in the simple view and priced in the advanced one.
- VAT and fees percentages (advanced): override the 20% and 15% assumptions. VAT is 20% on standard extensions (zero-rating applies only to some conversions and new builds), and a 10% to 15% fees-and-contingency layer is normal.
- Kitchen and bathroom fit-outs (advanced): the kitchen inside a kitchen extension is its own project, easily GBP 10,000 to GBP 25,000, and a bathroom or WC adds GBP 4,000 to GBP 10,000.
- Party wall awards (advanced): where foundations approach the neighbours, formal awards cost about GBP 1,500 per adjoining owner in surveyor fees. The friendly-notice route, where a neighbour consents in writing, costs nothing.
- Planning route (advanced): permitted development covers a generous envelope of rear single-storey extensions free of charge; a householder planning application in England costs GBP 528, and conservation areas usually need one.
- Grand total and cost per m2 (advanced): everything added together, then divided by the floor area actually gained (footprint x floors), which is the figure to set against moving costs or a loft conversion.
- Procurement: three itemised like-for-like quotes, a JCT-style contract with staged payments and a close read of provisional sums protect the budget more than any rate table.