Extension Builders: Fixed Price vs Day Rate (2026)
Builders price an extension two ways. A fixed price (lump sum) is one agreed figure for the whole job — the builder carries the risk. A day rate means you pay for time worked, which suits small or unpredictable jobs but shifts the risk to you. For most full extensions a fixed price wins on cost certainty; a day rate has its place for the genuinely unknown parts. Here is how they compare, with typical 2026 UK rates and how to make either one safe.
The two ways builders price an extension
Almost every extension quote is one of two shapes. Understanding which you are being offered — and why — is the difference between a build that lands on budget and one that drifts.
Fixed price (lump sum)
One agreed figure for the whole job against a defined specification and drawings. The builder prices the risk once and carries any overrun, so your number is firm unless you change the scope. This is the right default for a full extension.
Day rate (time and materials)
You pay an agreed rate per day plus materials. It is fair for work nobody can price up front — groundworks on an unknown site, repairs, or finishing tasks — but the risk of it taking longer sits with you, so it needs close monitoring.
At a glance: fixed price vs day rate
| Fixed price (lump sum) | Day rate | |
|---|---|---|
| Cost certainty | High — one agreed figure | Low — depends on days worked |
| Who carries the overrun risk | The builder | You, the homeowner |
| Best for | Well-defined whole builds | Small or unpredictable work |
| Typical use on an extension | The main build | Groundworks, repairs, finishing |
| Builder’s motivation | To work efficiently | Paid regardless of speed |
| What it needs from you | A clear spec & drawings up front | Weekly monitoring of hours |
Neither model is a trick — each is honest for the right job. The mistake is accepting a day rate for a whole, well-defined extension (open-ended cost) or forcing a fixed price on work that genuinely cannot be defined yet (padded to cover the unknown).
Typical trade day rates — 2026
What a day rate actually buys per trade. A general builder co-ordinates the others on most extensions.
Source: BestBuilders 2026 quote data · 519 UK towns. See our builder day rate guide.
Day rates and fixed-price bands (2026)
| Item | Typical cost (2026) | Notes |
|---|---|---|
| General builder day rate | £180–£280 per day | Higher in London and the South East |
| Two-person team day rate | £300–£480 per day | Builder plus labourer |
| Half-day call-out | £120–£200 | Small finishing jobs |
| Fixed price, single-storey extension | £1,800–£2,600 per m² | Standard finished spec |
| Fixed price, typical 20m² build | £36,000–£52,000 | See our extension cost guide |
| Day-rate allowance for unknowns | cap at 10–15% of the build | Only for genuinely undefinable work |
| Typical up-front deposit | 10–15% max | Never a large cash sum |
Regional price differences
| Region | Against UK average |
|---|---|
| London | +20–30% (labour, access, parking) |
| South East & commuter towns | +10–20% |
| Southern & Eastern England | +5–15% |
| Midlands | Around the average |
| North West, Yorkshire | −5–10% |
| North East, South Wales & Scotland | −10–15% |
Day rates and fixed prices move together by region, so compare local quotes against local rates rather than a national headline.
When a fixed price wins
- The job is well defined — you have drawings and a written specification.
- You want cost certainty and a number you can plan a mortgage or savings around.
- You will not be on site to supervise daily.
- You are comparing several builders — a fixed price on identical drawings makes quotes truly comparable.
- Most full extensions fall here — it is the sensible default.
When a day rate wins
- The work cannot be defined yet — groundworks on an unknown site, hidden repairs, awkward alterations.
- It is a small or short task where pricing a fixed figure is more hassle than it is worth.
- You have a builder you trust and can check progress regularly.
- You are happy to carry the timing risk in exchange for not paying a risk premium baked into a fixed price.
Making either one safe: your checklist
- Get everything in writing — the price basis, the specification and the drawings it is priced against.
- For a day rate, agree a not-to-exceed cap and the expected number of days.
- Ask to see the builder’s public liability insurance and who signs off Building Regulations.
- Agree a stage-payment schedule — never more than 10–15% up front.
- Keep a 10–15% contingency back whichever model you use.
- Compare three quotes on the same drawings, and read our guide to choosing an extension builder.
Red flags
- A day rate for the whole build with no cap and no estimate of days — open-ended cost.
- A “fixed price” with no specification or drawings attached — it will not hold.
- Large cash deposits or “VAT-free” offers — no protection if it goes wrong.
- Reluctance to put the pricing basis in writing.
- A fixed price far below the others — something is missing from the scope.
Common Questions
Related Guides
More on choosing and budgeting an extension.