Compare Extension Builders: How to Check Reviews in 2026 (UK)
Picking the wrong extension builder is the single most expensive mistake in a home project — a stalled or botched single-storey rear extension can cost £15,000–£40,000 to put right. The fix is due diligence before you sign. This 2026 guide shows you exactly where to find genuine reviews (Google, Checkatrade, Trustpilot, the BestBuilders reviews page), how to spot fake reviews, the insurance and accreditations to verify (FMB, TrustMark, public liability), the questions to ask referees, and a 10-point scoring checklist so you can compare three builders like-for-like and hire with confidence.
How do you check an extension builder’s reviews properly?
- Read across at least three platforms — Google Business Profile, Checkatrade and Trustpilot — never trust a single 5-star page in isolation.
- Ignore the star average, read the 3-star reviews. The most honest signal is how a builder handled a job that went slightly wrong.
- Spot fake reviews: a cluster posted in the same week, generic first-name-only profiles, no project detail, and no owner replies are all red flags.
- Verify accreditations independently — check the FMB and TrustMark registers directly, don’t just trust a logo on a website.
- Always ask for two recent extension referees and actually phone them — a genuine builder will hand these over without hesitation.
- Score three builders on the 10-point checklist below so you compare reputation, not sales patter.
“Checking reviews” in 2026 means more than glancing at a star rating. Extension builders are among the most reviewed — and the most fake-reviewed — trades in the UK, because the jobs are high-value and the competition is fierce. A convincing set of five-star testimonials on a builder’s own website means little; what matters is independently hosted reviews you can’t edit, accreditations you can verify on a public register, and referees you can actually speak to. This guide walks through each source, what it’s worth, and how to weigh them into one confident hiring decision.
Where to Find Genuine Extension-Builder Reviews: 5 Sources Compared
No single platform tells the whole story. Here’s how the main UK review sources stack up for judging an extension builder in 2026 — and how much weight to give each.
Rule of thumb: give the most weight to sources you can’t edit and that tie feedback to a real, dated job — Google, Checkatrade, Trustpilot and BestBuilders. Treat a builder’s own testimonials page as a portfolio, not evidence.
Red Flags: How to Spot Fake or Bought Reviews
The Competition and Markets Authority made fake reviews explicitly illegal under the Digital Markets, Competition and Consumers Act, which took effect in 2025 — but they haven’t disappeared. Here’s a scoring grid to sanity-check a suspiciously glowing profile.
The single most useful move: read the negative reviews first. Every established builder has a couple of three-star jobs — that’s normal and healthy. What you’re judging is the response. A builder who replies calmly, acknowledges the issue and explains how it was resolved is showing you exactly how they’ll behave when something goes wrong on your extension. A profile with zero criticism, or with hostile replies, tells you more than any number of glowing testimonials.
Verify These Before You Shortlist Anyone
FMB membership
Federation of Master Builders. Independently vetted and offers an insurance-backed warranty via FMB Insurance. Search the builder on the FMB “Find a Builder” register directly — don’t trust a logo alone.
TrustMark registration
Government-endorsed quality scheme. Confirms the builder meets standards for competence, customer service and trading practices. Verify on the TrustMark register at trustmark.org.uk.
Public liability insurance
Minimum £2m cover is standard for extension work. Ask to see the certificate and confirm it’s in date — if scaffolding falls or a wall is undermined, this is what protects you.
Companies House record
Check how long the company has traded, whether it’s been dissolved and reformed under a new name, and who the directors are. A pattern of phoenix companies is a serious warning sign.
Building control & warranties
Confirm they work with Building Control sign-off and offer a structural warranty on the extension. No warranty on a £40k extension is a deal-breaker.
Two recent referees
Ask for two homeowners whose extensions completed in the last 12 months — and phone them. A builder who won’t provide references has something to hide.
The 5 Questions That Reveal the Truth
Reviews tell you the headline; a five-minute call with a past customer tells you the reality. Ask these five and listen for hesitation.
1. Did they finish on the quoted budget?
The single biggest source of extension disputes is cost creep. Ask what the final bill was versus the quote, and whether every variation was agreed in writing before the work happened.
2. Did they finish roughly on time?
A few weeks’ slip on an extension is normal; a job that ran three months over with the crew disappearing to other sites is a red flag about how they manage workload.
3. How did they handle problems?
Every extension hits a snag — ground conditions, a steel spec change, a delayed supplier. What matters is whether the builder communicated early and fixed it, or went quiet.
4. Was the site kept safe and tidy?
Site tidiness is a proxy for professionalism. Ask whether waste was cleared, scaffolding was secure, and the family could live around the works safely.
5. Would you hire them again?
The closing question. A confident, immediate “absolutely” is worth more than any star rating. A pause, a qualified answer, or a “well…” is your cue to dig deeper.
Bonus: can I see the finished extension?
Genuine happy customers are usually glad to show off their new space. A builder who can arrange a viewing of recent work is showing you real, standing proof — not a photo that could be anyone’s.
The 10-Point Builder Scorecard
Score each shortlisted builder out of 10 on the factors below (0–2 points each where marked), then compare the totals. Anyone under 12/20 should be treated with caution.
Run all three quotes through this scorecard side by side. If one builder is cheaper but scores 10/20 on reputation while another scores 18/20, the “expensive” quote is almost always the cheaper choice once you factor in the cost and stress of a job going wrong. Browse verified BestBuilders reviews to see how our matched builders score.
Common Questions
Related Guides
More cost, planning and how-to guides to help you plan your extension and hire the right builder with confidence.
Single-storey extension cost 2026
Full 2026 cost breakdown so you know a fair quote when you see one.
Read Guide →How to plan a house extension
Step-by-step planning checklist before you approach any builder.
Read Guide →BestBuilders verified reviews
Read genuine, job-verified reviews of the extension builders we match.
Read Reviews →Get 3 Free Extension Builder Quotes
BestBuilders matches you with 3 vetted extension builders — each FMB or TrustMark checked, insured, and reviewed — who price your project against a fixed brief so you can compare like-for-like. Insurance-backed warranty included.