A builder’s quote, read line by line
Two quotes for the same extension, £26,000 apart. This is what an independent builder sees in the cheaper one — and it is rarely the price that is the problem.
Illustrative example, not a real customer’s job. The situation is typical and the numbers are the ranges we work to — but nobody’s quote, photos or address end up on a website.
This is not a bad builder, but it is an incomplete quote. Three things that will definitely be needed are not in it, which is most of the gap to the dearer quote. Get those priced before you compare the two, and change the payment schedule before you sign anything.
What the quote said: “Excavate and pour foundations as required — provisional sum £4,500.”
A provisional sum is a placeholder, not a price: whatever it actually costs is what you pay. Foundation depth near the boundary and the trees at the bottom of the garden is the single biggest unknown on this job. Ask for the depth the quote assumes and the rate per extra 300mm, in writing. That turns an open cheque into a number you can plan for.
Typically £1,000–£6,000 over a shallow assumption
The drawing shows the back wall coming out, so there is a beam. The quote prices the brickwork and the opening but nothing for the engineer’s design, the steel itself, the padstones or the fire protection. The dearer quote has all four. That is not the other builder being expensive.
Engineer £400–£900 · steel, padstones and protection £1,200–£3,500
Your photo 3 shows the stack on the rear wall. Nothing in the quote moves it, and nothing mentions a build-over agreement with the water company if the run passes near the sewer. Both are normal and neither is expensive — but they are weeks, not days, if they turn up mid-build.
Move the stack £1,800–£4,500 · build-over application £300–£600
Half the job’s value before anyone has stood on site is not normal and is the single most common way people lose money. A reasonable schedule on a job this size is a small mobilisation payment for materials, then stage payments against work that has actually happened — slab down, walls up, roof on, plastered, snagged. Payment should always sit slightly behind the work, never ahead of it.
The brickwork, roof and plastering rates are sensible for the region and the finish you described, the programme of 9–11 weeks is realistic rather than flattering, and it names who is doing the work. Once the three missing items are added, the two quotes are within about £3,000 of each other — which makes this a decision about the builder, not the number.
- Send both builders the same three questions: assumed foundation depth and the rate beyond it, the steel and engineer line, and the drainage.
- Ask for the payment schedule to be re-issued as stage payments against completed work.
- Check the company at Companies House and ask for two addresses of jobs of the same size finished in the last year.
- Confirm who is applying for building control, and whether the quote includes the fee.
- Only then compare the totals — and expect them to be much closer together.