Extension Quotes: Fixed Price vs Day Rate — Which Wins in 2026?
Two builders quote your single-storey rear extension. Builder A offers £78,000 fixed price. Builder B offers £320/day day rate and a 14–18 week estimate. The day-rate maths is £28,000–£38,000 cheaper on paper — but the comparison isn't apples-to-apples. Fixed price moves the risk to the builder; day rate keeps it on you. Here's how to read each quote and which model genuinely wins by project type in 2026 UK.
Fixed Price vs Day Rate: What Each Model Actually Is
Fixed-price quote
A single inclusive figure for the full scope, broken down into stage payments (typically 5–7 stages: groundworks, frame & roof, first fix, second fix, finishes, completion). Builder absorbs labour overruns. You absorb only PC-sum overruns (kitchen, tiles, sanitaryware) and genuine variations.
2026 typical: £52,000–£120,000 for single-storey rear extension
Day-rate quote
A daily rate per trade (£280–£360 main builder, £300–£380 electrician, £350–£420 plumber, £220–£300 labourer) plus materials at cost + 10–20% markup, plus an estimate of duration. You pay for time taken, not output produced. Overruns are your bill.
2026 typical: £34,000–£95,000 actual outturn for the same extension
Who Carries the Risk — And What It Costs
| Risk | Fixed price | Day rate |
|---|---|---|
| Bad weather delay | Builder | You |
| Slow trades, missed appointments | Builder | You |
| Unknown ground conditions | Often shared | You |
| Material price rises mid-build | Builder (usually) | You |
| You change scope mid-build | You (priced variation) | You (more days) |
| Building control rework | Builder | You |
Fixed price has a built-in risk premium: the builder marks up to cover the worst plausible week of the build, then pockets the difference if it doesn't happen. The premium is typically 8–18% over the median outturn cost of a day-rate equivalent — which is exactly the price of insurance.
Which Quote Model Wins By Project Type
Fixed price usually wins
- Single-storey or double-storey rear extension, fully specified drawings
- Project >£50k or >8 weeks duration
- You can't supervise daily or want clean monthly stage payments
- You're financing on a mortgage release that needs predictable draws
- First-time builder relationship (no track record yet)
Day rate usually wins
- Small extension <£35k or refurb-style work where scope evolves on site
- You have an existing trusted builder relationship
- You're on-site yourself daily (architect, project manager, retired tradesman)
- The job has high unknown content (Victorian terrace strip-out, listed building)
- You explicitly want optionality to upgrade spec mid-build
How To Compare Quotes Like-For-Like in 2026
- Normalise both quotes to a single inclusive figure. Convert the day-rate quote into expected outturn = (days × rate) + materials + markup + 15% contingency.
- Audit the prime-cost sums. A fixed-price quote with a £2,500 PC sum for the kitchen is hiding a future variation. Require PC sums to be realistic for the spec you want, or replaced with named-supplier figures.
- Check the payment schedule. Fixed-price quotes that ask for >25% before any work is on site are red flags. Stage payments should track value-of-work-completed.
- Confirm warranty and snagging window. Reputable fixed-price builders offer 12 months snagging plus structural insurance-backed warranty. Day-rate has no implicit warranty — every defect is a fresh day to rectify.
- Ask for two recent references with photos. Same scope, same region, same calendar quarter. Phone the references; visit one in person if you can.
Same Extension, Two Quote Models — What Happened
A 30 m² single-storey rear extension in St Albans, spring 2026. Same drawings, same spec.
Builder A · £84,500 fixed
14-week programme, 6 stage payments. Final cost: £87,300 (one variation — client upgraded bifold doors from PC sum). 12-month snagging visit included. Programme overran by 8 days at no cost to client.
Builder B · £340/day estimated 16w
Estimated outturn £66,000–£74,000. Actual outturn: £92,400. Reasons: 4 weeks of weather delay billed at full rate, two material price rises, three weeks of programme drift on second fix. Client had no contractual recourse.
The day-rate quote was £18,500–£28,500 cheaper on paper. Actual outturn was £5,100 more expensive than the fixed price — and the client lost six weeks of their own time to project chasing. This is the typical 2026 pattern on jobs above £50k.