How to choose a London builder: a 9-point assessment framework

The London residential construction market has a wide range of contractors — from established design-and-build firms with hundreds of completed projects to one-man-band cowboys with a van and a Facebook page. The price difference between the top and bottom of the market is often less than people assume; the quality and risk differential is enormous.

Over 60% of the complaints handled by Trustpilot, Houzz and the FMB in the London residential category in 2024 involved contractors that the client could have identified as high-risk before signing. The information was available — but the client didn't know what to look for.

1. Legal entity. Is the contractor a limited company with Companies House filings showing trading history over 5+ years? One-person sole traders are not necessarily cowboys but the structural risk is higher — no separation between business and personal liability, no auditable accounts, no defined ownership.

2. Public liability insurance. Minimum £5M, ideally £10M. Ask for the certificate, check it's current and the named insured matches the trading entity.

3. Employer's liability insurance. Required if they have any employees. Same checks as above.

4. Professional indemnity insurance. For any design-and-build contractor (i.e., one supplying design work as well as construction). Minimum £1M, ideally £2–5M. Catches design errors that lead to construction defects.

5. Membership of a recognised body. FMB (Federation of Master Builders), TrustMark, NHBC registered builder, or similar. Membership is no guarantee but absence is a red flag.

6. References. Ask for 3 client references from completed projects, ideally recent and in your borough. Phone or visit one. Cowboys can't produce three.

7. Pricing structure. Fixed-scope written quote with itemised line items, not 'roughly £X' or '£Y/day'. Provisional sums must be transparent and limited.

8. Contract document. JCT Minor Works or similar industry-standard contract for projects over £25k. Cowboys often work off a single-page 'agreement' which provides little protection.

9. Structural warranty offered. 10-year insurance-backed warranty for major works. NHBC, Premier Guarantee, LABC, BLP, ICW or Buildzone are the recognised providers. No warranty = no protection past defects period.

Cash discount offers (suggests undeclared income; also no recourse if work is poor). Vague written quotes (lots of provisional sums, day rates, 'allowance' line items). Refusal to provide insurance certificates. No accessible Companies House record. Recent change of trading name (often hides previous complaints). High deposit demands (40%+ upfront for materials is unusual and often a cashflow problem the client absorbs). Pressure to sign quickly before exploring alternatives. Refusal to use industry-standard contract documents.

Any single red flag is a flag, not a stop sign — context matters. Two or more red flags is a stop. Don't proceed.

Above the basic compliance bar, evaluate fit. Did the senior person turn up to the site survey, or just a salesperson? How responsive were they to follow-up questions in the days after the survey? Did their quote demonstrate genuine understanding of your specific property, or could it have been written for any London terrace?

Construction is a long-relationship business. You'll spend 12–36 weeks (depending on project) interacting with the contractor's people daily. Cultural fit matters — communication style, decision-making process, attitude to small problems. These are unmeasurable on a checklist but become very visible in the first 30 minutes of a site survey. Trust your gut alongside the checklist.