Why your contractor's quote is probably wrong (and how to fix it before you sign)
Three structural reasons a builder's number rarely matches the finished cost, and the cost expert checks that close the gap before you sign.
Key facts
- Contractor quotes typically understate measured quantities by 5–15% on residential projects.
- Provisional and PC sums are placeholders, not budgets, they are expected to increase.
- An independent cost check before signing costs 0.5–1.5% of project value.
- The cheapest quote wins about 40% of the time but produces the highest closing out the build in 60% of cases (Costed observed).
- Comparing three quotes against a single Priced Spec is the only true like-for-like comparison.
A contractor's quote is a marketing document as much as a financial one. It is competing against other quotes, against the homeowner's budget, and against the contractor's own desire to win the job. None of those pressures push the number upward.
The cheapest quote is almost never the cheapest project.
There are three structural reasons a quote will move once work begins. Address all three before you sign and the gap between quote and outcome narrows dramatically.
1. Undefined specification
If the spec doesn't name brands, finishes, and standards, the contractor has priced for the cheapest compliant version of each item. The variation to upgrade is yours to pay.
Specifics that need pinning down before pricing: kitchen carcass and worktop spec, sanitaryware brand and model, ironmongery, paint grade, floor finish and underlay, tile thickness and grout, glazing U-value and frame finish, structural opening details, fire-rated door cores, M&E equipment models.
This is not pedantic, every undefined item is a future variation in the contractor's commercial favour.
2. Optimistic measurement
Quantities taken from drawings by eye, rather than measured to, routinely understate by 5–15%. A 200 m² extension priced on a 180 m² estimate looks competitive at quote and reappears as a £30,000 variation in month three.
The fix is to issue a measured Priced Spec and have contractors price the same scope, or, for smaller projects, to commission an independent take-off and compare to the contractor's figures before signing.
3. Provisional and PC sums treated as budgets
A "PC sum" or "provisional sum" is a placeholder for work that cannot be priced at quote. your build contract contracts require it to be adjusted at closing out the build against actual cost. The mistake is treating these placeholders as fixed budgets.
If a quote contains £15,000 as a PC sum for the kitchen, the kitchen probably will not cost £15,000, it will cost what it costs, and the difference is yours. A cost expert reading the quote flags every PC and provisional sum, tests whether the placeholder is realistic, and either firms it up or holds explicit contingency against it.
The independent cost check
A pre-signature cost check is the single highest-leverage hour you can spend on a domestic project. The output is a one-page summary covering:
- Items missing from the quote
- Rates that sit outside the current London market range
- Quantities that look light against the drawings
- Provisional sums that need firming up
- Contract terms (retention, LADs, variation procedure) that need negotiating
For most projects under £500,000, this check costs £900–£2,500 and is the difference between a project that finishes on budget and one that does not.
Frequently asked
- Should I always take the cheapest contractor quote?
- No. The cheapest quote is almost never the cheapest project. It usually means the contractor has priced the cheapest version of an undefined spec, light quantities, or aggressive provisional sums. Compare quotes against a single Priced Spec and the cheap one often disappears.
- What is a provisional sum?
- A placeholder cost for work that cannot be measured at quote, typically because the specification is not yet final (kitchens, M&E equipment, specialist works). PC and provisional sums are adjusted at closing out the build against actual costs incurred. Treat them as guaranteed variations, not budgets.
- How do I know if a contractor's measurements are accurate?
- You don't, unless an independent cost expert measures the same drawings to and compares. On almost every residential job, the contractor's measure is 5–15% light. The shortfall reappears as variations once work starts.
- What is a 'fair' contractor markup on a domestic project?
- Overheads and profit (OHP) on a residential project typically run 12–22% of trade cost, lower on large jobs, higher on small jobs and heritage work. OHP should be transparent on the priced bill, not buried in unit rates.
- How long does an independent quote review take?
- Three to five working days for a typical residential project once we have the contractor's priced quote and the drawings. The output is a one-page summary identifying missing items, rate outliers, optimistic quantities, and unsigned provisional sums.