Builder Quote Sanity Checker

If a builder has quoted for your extension and you have no idea whether the number is fair, the honest answer is that the figure alone tells you very little. What matters is the scope behind it. A full single-storey build runs around £2,200 – £2,500 per m² before the kitchen, and £2,800 – £3,000 per m² for premium finishes or the South East, but a builder's quote is usually only part of that total. This checker turns the quote into a scope question: tick what the quote covers, and see whether the price is realistic for that work, and what has been left out.

What a builder's quote often leaves out

The trap with a headline quote is assuming it is all-in. Very often it covers the structure the builder controls directly, foundations, walls, and roof, and stops there. Structural steels, windows and external doors, electrics, plumbing, tiling, and decoration are then billed as extras, sometimes by separate trades you have to find and manage yourself.

Two further items distort comparisons. A provisional sum is an estimated allowance for work that cannot be priced exactly yet, and it will move up or down once the real cost is known. And a builder under the VAT registration threshold of £90,000 does not add 20% to their invoices, so a sub-threshold quote can look materially cheaper for identical work. Normalise every quote to the same scope and to a VAT-inclusive figure before you trust the totals.

How the checker builds its estimate

The tool holds a baseline rate for each trade category, calibrated against a documented South East build, then scales it by your floor area, region, and specification level. London and the South East sit at the top of the range; the North, Scotland, and Wales run lower. Basic spec trims each rate and premium lifts it. Tick the categories your quote covers, enter the quoted figure, and the checker returns a traffic-light verdict for the included scope plus an estimate of what the excluded items will add.

The result reads against the cost per m² benchmark rather than one lump total, so you can see where a quote is thin and where it is fair.

A worked example

Take a 30m² single-storey rear extension in the Midlands at mid-range spec. The checker starts each trade from its baseline rate, foundations at £140 per m² and structural steels at £75 per m², scales them by size and region, and totals the result to show a typical range for the scope you selected.

Say the builder's quote lists only foundations, walls, and roof. The checker flags the missing lines, steels, windows, electrics, and plumbing, and estimates what each will add, so the headline figure stops looking like a complete price. On one documented build the builder's quote covered only foundations, walls, and roof while steels, windows, electrics, plumbing, tiling, and decoration were all additional, which is exactly the gap this tool is built to expose. See getting and comparing builder quotes for how to turn that into a fair like-for-like comparison.