What Is a Provisional Sum?
Written by James Walker · Last reviewed 15 August 2026
James Walker has 2 degrees in construction (BSc (Hons) Building Surveying · MSc Environmental Design of Buildings) and an extensive background in issue resolution, resolving over £4 billion of consumer issues.
A provisional sum is a figure included in a quote for work that cannot yet be priced accurately, because the extent is unknown until it is opened up. It is adjusted up or down once the real cost is established.
Why they exist
Some things genuinely cannot be priced in advance. Foundation depth, the condition of hidden timber, the state of pipework under a floor. A builder who quotes a firm price for these is either guessing or building in a large margin.
A provisional sum is the honest way to handle that uncertainty.
What to watch
A quote containing several large provisional sums is not really a quote. The headline figure looks competitive and the final cost is largely undetermined.
Ask how each provisional sum was arrived at, what would make it go up, and how the final figure will be evidenced.
How they should be settled
By evidence, actual quantities, actual invoices, actual time. Agree in advance how it will be measured, so the adjustment is a calculation rather than a negotiation.
How it works
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