Compare Extension Quotes UK 2026: What to Look For
Three quotes for the same extension can vary by 60%. The cheapest is rarely the best value — but the most expensive is rarely the safest either. This 2026 UK comparison playbook gives you the 12-line audit that separates a fixed-price contract you can sue on from a marketing leaflet dressed as a quote.
What a Good Extension Quote Looks Like in 2026
✅ Solid Quote Has
- Itemised stages — strip-out, foundations, frame, M&E, finishes
- Fixed price (or clearly labelled provisional sums with caps)
- VAT explicit — included or excluded
- Structural engineer fee shown
- Building Control fee included
- Public & product liability insurance disclosed
- Stage payments tied to milestones, not weeks
- FMB / CIOB / TrustMark badge with verifiable number
❌ Walk Away If
- One-page lump-sum quote with no breakdown
- 50%+ deposit demanded upfront
- "VAT TBC" or VAT not mentioned at all
- No structural engineer line — they will ask later
- No insurance certificates available
- "Allowance" lines without quantities
- No written specification document
- No final account / variations clause
What "Cheap, Mid, High" Look Like in 2026
| Single-storey rear extension | Cheap | Mid | High |
|---|---|---|---|
| Per m² (UK 2026) | £1,800–£2,200 | £2,400–£3,000 | £3,200–£4,500+ |
| Risk profile | High variation, scope creep | Mainstream, predictable | Low risk, premium finish |
| Typical contract | Letter quote | JCT Minor Works | JCT Intermediate |
| Final price drift | +15–30% common | +5–10% typical | <5% |
How to Compare Three Quotes Like-for-Like
1. Same drawings, same spec
Send all three builders the same drawings + 1-page specification (kitchen included? glazing spec? floor finish?). 80% of price variation traces back to scope, not labour.
2. Strip out provisional sums
Provisional sums are placeholders that get re-quoted later. Compare the fixed portion of each quote, then add provisional sums separately.
3. Check insurance & warranties
FMB MasterBond and CIOB Construction Online Pro warranties cover defects up to 10 years. A £200 warranty premium is the cheapest insurance you'll buy.
4. Reference the last 3 jobs
Ask for the last 3 completed jobs and visit one. The quote that wins on paper but fails on referencing is the one to drop.
Get 3 Apples-to-Apples Extension Quotes
BestBuilders sends the same drawings + spec to up to 3 vetted UK builders so you can compare prices on the same scope.