What Is a Prime Cost 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 prime cost sum, usually shortened to PC sum, is an allowance included in a quote for an item you have not yet chosen. If your final choice costs more, you pay the difference; if less, you are credited.
Where you will meet it
Kitchens, bathroom suites, tiles, sanitaryware, ironmongery, flooring. Anything where the builder needs a number to complete a quote but the actual product is your decision.
Why it causes friction
Because the allowance is often set low. A quote looks competitive because it assumes a modest figure for tiles, and you then choose tiles at three times that.
The difference is legitimate, but it lands late, when the budget is already committed, and it feels like a price increase rather than a choice you made.
How to protect yourself
Ask what each PC sum assumes, in real terms. Not "£40 a square metre" but "tiles at this sort of level". Then go and look at what that actually buys before accepting the quote.
If you already know what you want, price it and get the allowance set correctly from the start.
How it works
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