What is a prime cost sum, and who pays the difference?

A prime cost sum (PC sum) is an allowance in your price for the cost of a specific thing that has not been chosen yet: the client knows they want a kitchen, tiles, ironmongery, but has not picked which, so the contract carries a stated figure for it and everyone moves on.

How the adjustment is supposed to work

When the choice is made and the real cost is known, the allowance comes out and the actual cost goes in. Suite costs £5,400 against a £3,000 allowance: the contract sum rises by £2,400. Suite costs £2,100: it falls by £900, and yes, that cut is as binding as the increase, which is the half of the rule builders forget they signed up to.

Two things ride on top and are yours to protect:

• Your addition for profit and attendance. Priced forms typically carry the contractor’s percentage on PC sums as its own entry. When the sum adjusts, your percentage adjusts with it, if you priced one. If your quote shows a bare PC figure with no stated addition, you fit and coordinate the client’s £5,400 suite for nothing, and no adjustment will invent the margin you never wrote down.
• The fixing was never in the PC sum. The sum covers the cost of the goods; the labour to install them lives in your measured work. Keep saying so, because clients read “PC £3,000” as “bathroom, all in”.

Not the same thing as a provisional sum

The neighbouring term, and the confusion costs money. A PC sum is a known scope with an unknown product price: there will be sanitaryware, we know who fits it, we do not know which suite. A provisional sum is an allowance for work that is not yet defined at all, and when it firms up it gets valued like a variation, potentially with time consequences. The distinction has its own page; the practical test is whether the unknown is a price (PC) or a scope (provisional).

On a job for someone living in the house

PC sums are pure contract machinery, so they work on a domestic job exactly as written, and this is a page where writing matters more than law. In a quote to a homeowner, spell each one out in words a non-builder reads correctly: the allowance, what it covers, that the final cost replaces it up or down, and your percentage on top. A client who chose the £5,400 suite knowing the allowance was £3,000 pays the difference without a fight; one who finds out at the final account fights, and on consumer terms an ambiguity will generally be read against the person who drafted it, which is you.

What to do this week

1. List the PC sums on your live job and check which have already been overtaken by real choices with no adjustment run.

2. Put every crystallised adjustment in the next application, with the invoice for the actual cost attached.

3. Check your quote template: every PC sum should state its addition percentage and say the fixing is measured elsewhere. Two sentences that end most PC arguments before they start.

Where the information stops

How your particular form words the adjustment, and what your stated percentage does and does not ride on, is a reading of your documents; on a dispute over a large adjustment, especially one tangled with variations, that reading is worth a professional hour before you fix a figure in writing.