Compare · Updated May 2026
Compare Extension Builders: How to Vet Quotes in 2026 (UK)
A side-by-side extension quote comparison in 2026 UK should never be a one-line price fight. The cheapest tender is almost always missing £8,000–£18,000 of provisional sums, exclusions or programme contingency that the more expensive tender quietly includes. Run all three quotes through the 11-point checklist below and the real ranking flips on roughly two jobs in three.
11-point vetting checklist Apples-to-apples comparison Avoid the £15k underbid trap
The 11-point quote vetting checklist
Print this. Stick it on every quote you receive. Each missing line is £1,500–£6,000 of risk you’re absorbing without realising.
- Specification cross-reference. Each line on the quote should map to a page on your architect’s drawings and to a section of the Building Regs spec. If a quote just says “extension as per plans” without breaking down structure / shell / first-fix / second-fix / finishes, it’s not comparable to anything.
- Insurance certificates — current. Public liability £5m minimum, employer’s liability £10m, and contract works insurance covering the value of the build. Ask for the certificates, check the expiry date, phone the broker if you’re unsure.
- Trade-body membership. FMB, CIOB, NHBC, or Chartered Builder. Each one means the firm has been audited and carries professional indemnity. Membership without an expiry date you can verify is worthless.
- Programme & start date. A realistic single-storey rear extension is 12–18 weeks; double-storey 18–26 weeks. Anyone quoting under 10 weeks for a rear extension is either over-promising or under-pricing labour.
- Payment schedule. Front-loaded deposits over 15% are a red flag. Standard 2026 schedule: 10% deposit on signing, 20% on completion of foundations, 25% on completion of shell, 25% on completion of first fix, 15% on practical completion, 5% retention released at 6 months.
- Provisional sums. Items like kitchen, sanitaryware, tiling, flooring, internal doors should be listed as PC sums with a stated £ budget. If the quote rolls them into a single “finishes” line you cannot compare across tenders.
- Exclusions. Every quote excludes something — the question is what. Common 2026 underbid exclusions: ground conditions, asbestos survey, party wall surveyors, drainage diversions, scaffolding past week 12.
- VAT clarity. Confirm whether the quoted price is plus VAT or inclusive. On a £75,000 build that’s a £15,000 difference.
- Warranty. 10-year insurance-backed guarantee (IBG, FMB-MasterBond, or QANW) is the 2026 baseline. A “builder’s own warranty” means nothing once the builder dissolves.
- Project manager named. Who is on-site every day? Who is your single point of contact? If the quote can’t name them now, you’ll be chasing different people every week.
- References — 3 recent & visitable. Phone two, visit one. Ask the homeowners about timeliness, cleanliness, defects rectification, and whether the final invoice matched the quote.
How the cheapest quote becomes the most expensive
| Item | Builder A (low) | Builder B (mid) | Builder C (high) |
|---|---|---|---|
| Headline quote | £68,000 | £79,500 | £84,000 |
| Kitchen PC sum included? | No (+£9,000) | £7,500 inc. | £9,000 inc. |
| Sanitaryware? | No (+£2,400) | £2,000 inc. | £2,500 inc. |
| Party wall fees? | Excl. (+£2,200) | Incl. | Incl. |
| True comparable cost | £81,600 | £79,500 | £84,000 |
Builder A’s £68k quote ended up the second most expensive once you adjust to a like-for-like spec. Builder B was the real-world cheapest. This is the most common pattern across 2,100+ extension quotes BestBuilders compared in 2025–2026.
Red flags that should kill a quote on sight
- Cash-only payment, or deposits paid into a personal bank account
- No written quote, only verbal “trust me, mate” figures
- Quote on a single side of paper with no line-item breakdown
- Pressure to sign today / “this price expires tomorrow”
- Trade-body logos on a website with no membership number you can verify
- Refusal to share insurance certificates
- References all from the past 6 weeks (no jobs older than that = brand-new firm or repeat dissolutions)
FAQs
On builds above £120,000, yes — a QS day rate (~£650) typically uncovers £5k-£15k of buried risk and gives you negotiating leverage on PC sums. Below £120k, a careful walk through the 11-point checklist gets you 80% of the same protection for free.
It’s a strong signal but not the only one. A long-established sole-trader builder with five recent local references can be safer than a freshly-joined FMB member with one job under their belt. Use trade-body membership as a tiebreaker, not the sole filter.
Three is the right number. Two gives you a range too narrow to test. Four or more wastes everyone’s time and good builders learn fast that you’re shopping aggressively, which pushes the best names off your list.
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