How to Hire a Trusted Extension Builder in 2026 (UK)
A 9-step vetting checklist used by professional surveyors. Insurance and accreditations, references, contracts, payment milestones, retention and the warning signs that should make you walk away — protect a £50,000+ build with the same diligence a developer would.
The Builder Vetting Checklist (2026 UK)
Follow these in order. Skipping a step is how homeowners end up in dispute. Allow 2–3 weeks for the vetting phase — it is the cheapest insurance you will ever buy.
Define your project on paper first
Write a one-page brief: footprint, intended use, target budget, must-haves, nice-to-haves and the latest you want to be moved-in by. A defined brief is the only way to get like-for-like quotes. Vague briefs invite vague pricing and vague disputes later.
Verify insurance and check it is current
Ask for and read three certificates: Public Liability (£2m minimum), Employers' Liability (£5m minimum, legally required if they employ anyone) and Contract Works / Contractor's All Risks insurance covering your build value. Phone the broker on the cert to confirm the policy is live — a screenshot is worth nothing.
Confirm trade accreditations
FMB (Federation of Master Builders), CIOB (Chartered Institute of Building) and TrustMark are the meaningful UK badges in 2026. Verify membership numbers on the official website — not the builder's own. Gas Safe and NICEIC apply for any sub-contracted heating and electrical work.
Visit two finished projects — unaccompanied if possible
Photos lie, owners do not. Ask for the contact details of two recent customers, ideally finished within the last 12 months and one ongoing site. Visit at least one in person, ask the homeowner about timeline, change-control and how the builder behaved when problems arose.
Get three written, like-for-like quotes
Always at least three. Use the same drawing pack and specification for each. Treat £5,000+ outliers with caution — the cheapest is rarely the best, and the most expensive often reflects a tighter scope reading. Insist on a stage-by-stage breakdown.
Use a written contract with a fixed payment schedule
JCT Homeowner Contract, FMB Domestic Building Contract or a builder's own contract — reviewed by a solicitor (£300–£500 well spent). Specify start date, completion date, weekly hours, daily liquidated damages and a clear scope. Never start work on a verbal agreement.
Stage payments — not a deposit and a final cheque
Refuse a 30%+ deposit. Use 5–10 milestone payments aligned with verifiable on-site stages: foundation pour, oversite, weatherproof, first fix, plaster, second fix, decoration, completion. Hold a 5% retention for 6–12 months for snagging.
Insist on building control and a structural warranty
Either Local Authority Building Control or an Approved Inspector. Ask for a 6–10 year structural warranty (LABC, Premier, Buildzone) particularly if you may sell within a decade — mortgage lenders ask for them.
Manage variations and snagging on paper
Every change of mind goes on a written variation order with a price impact — same day, signed both sides. Keep a snag list throughout, not just at the end. Final payment only on completion of all snags. This is where 80% of builder disputes start.
Warning Signs That Should Stop You
⚠️ Cash-only or «no VAT» pricing
No paper trail = no protection. Indicates undeclared trading and almost no recourse if work fails.
⚠️ Pressure to start tomorrow
Reputable builders are booked 4–8 weeks ahead. Anyone available now is either new, between jobs or a problem.
⚠️ 30%+ deposit on day one
Working capital should not be your problem. Materials are paid for at first delivery, not pre-start.
⚠️ Reluctance to give a written quote
A verbal quote is meaningless when scope changes. If they cannot put a number on paper they should not be on your job.
⚠️ No fixed business address
A registered limited company with a Companies House address. Sole trader fine — but with a verifiable home address and trading history.
⚠️ Gut feeling tells you no
A 4–6 month build needs trust. If you do not feel comfortable on the survey, walk away — there are 519 towns full of alternatives.
What Vetting Itself Costs
Spending £500–£1,500 on diligence will save you 10× that on a typical UK extension build. The breakdown:
| Step | Time | 2026 cost |
|---|---|---|
| Solicitor contract review | 3–5 days | £300 – £500 |
| Quantity surveyor scope review | 5–7 days | £400 – £800 |
| Structural warranty | 2–3 weeks | £400 – £1,200 |
| References & site visits (your time) | 2 evenings | £0 |
Skip the Vetting With Pre-Vetted Builders
BestBuilders runs the first 3 stages of this checklist for every builder we list — insurance verified, accreditations confirmed, references checked. Get up to 3 vetted local extension specialists, free.