Why UK construction costs keep surprising clients (and what to do about it)
Three out of four homeowners are blindsided by the real cost of building work. Here is the honest breakdown — what is actually driving prices, what gets forgotten, and how to plan a budget that survives contact with reality.
By Build Hub Editorial · 8 May 2026
The £/m² lie
Most clients arrive with a number in their head — usually one they read in a Sunday supplement or heard from a neighbour. £1,800 per square metre. £2,400. £3,000 if it's London.
Those numbers aren't wrong. They're just incomplete.
A 2025 RICS member survey found regional rate variance of +38% to -22% against the UK mean — and that's before specification, access, ground conditions and the contractor market all weigh in.
The headline £/m² rate is a starting hypothesis, not a budget.
What actually drives the number
Five things move the needle more than people realise:
- Postcode — labour costs in the South East run roughly 12-28% above the UK average. Yorkshire and the North East run 10-12% below.
- Spec creep — kitchens and bathrooms swallow contingency. A £30k kitchen vs a £12k kitchen is the difference between "on budget" and "uncomfortable conversation".
- Ground & structure — the cheapest surprise on a UK extension is a 1.2m foundation. The most expensive is a 2.8m one.
- Programme pressure — fast-track jobs cost a premium. Always.
- Professional fees — architect, engineer, party-wall surveyor, planning consultant, principal designer. Budget 12-18% on top of build cost.
The line items everyone forgets
| Item | Typical cost on a £200k extension |
|---|---|
| CIL / s.106 contributions | £0 - £8k |
| Building Control fees | £900 - £2,100 |
| Party wall awards (per neighbour) | £900 - £2,500 |
| Skip and waste licences | £300 - £1,500 |
| Temporary kitchen / decant | £4k - £20k |
| Furniture and finishes | 8-15% of build |
Add it up and your "£200k extension" is a £240k project. That's not a contractor problem. That's a planning problem.
How to budget so you sleep at night
Use a three-layer model:
- Build budget — what the contractor actually quotes
- Project budget — build + fees + statutory + decant + finishes
- Headroom — 10% of project budget, ring-fenced
Then ask your contractor for a stage-by-stage drawdown schedule before signing anything. If they can't produce one in two pages, that tells you something.