Provisional sums and PC sums: using them without burning trust

A provisional sum is not a fudge and a PC sum is not a trap, but plenty of clients have been burned by builders who used them that way. Used properly, they are the honest answer to the parts of a job nobody can price yet. Here is how to use both without your final bill feeling like a stitch-up.

What the two terms actually mean

A provisional sum is an allowance for work that cannot be properly priced when the quote goes out, because the design is not settled or the extent is unknown. Rewiring an old house before you have lifted a floorboard. Underpinning that may or may not be needed. Drainage runs nobody has surveyed. You insert a realistic sum, flag it clearly, and price it properly once the facts arrive.

A PC sum (prime cost sum) is narrower. It covers the supply cost of goods the client has not chosen yet: the kitchen units, the sanitaryware, the facing bricks, the tiles. Your labour to fix those goods, and your overhead and profit on handling them, sit elsewhere in the quote as firm figures. Only the purchase price of the item itself floats.

The distinction matters because they move differently. A provisional sum can swing the whole package of labour, materials and plant. A PC sum should only ever move by the difference between the allowance and the till receipt.

Where trust gets burned

Clients do not resent provisional sums. They resent surprises. The damage happens in three predictable ways.

First, the lowball. A builder wants to win the job, so the provisional sum for the rewire goes in at half what any electrician would charge. The quote looks cheap, the contract gets signed, and three weeks in the client discovers the real number. Even if every penny is justified, it feels like bait and switch.

Second, the vague scope. "Provisional sum for electrical works" tells the client nothing. When the reconciliation lands, they cannot see what the allowance covered, so they assume the worst.

Third, the silent reconciliation. The job finishes, the invoice arrives, and the sums have quietly become different numbers with no conversation in between. That is how disputes start, and it is entirely avoidable.

Writing sums that protect both sides

Every provisional or PC sum in your quote should carry four things: a clear description of what it covers, the amount, what happens when the real cost is known, and who controls the decision. For example:

"PC sum for supply of wall and floor tiles: £1,200. Covers the purchase price of tiles only, based on an allowance of £40 per square metre for 30 square metres. Fixing, adhesive, grout and trims are priced separately and are firm. If your chosen tiles cost more or less, the difference is added or deducted at cost with the receipt provided."

Notice what that does. It sets an honest benchmark (£40 per square metre buys a decent tile, and the client can check that in any shop). It separates the floating part from the firm part. And it promises the receipt, which is the single cheapest trust-building device in construction.

For provisional sums, add the trigger: "Provisional sum for repairs to existing roof timbers: £800. The extent cannot be assessed until the roof covering is stripped. Within five working days of strip, we will provide a firm price for any repairs needed, for your approval before the work proceeds."

Common items and how to treat them

The last row is the discipline that separates careful estimators from lazy ones. A provisional sum is for the genuinely unknowable, not for the merely tedious. If you can measure it from the drawings, price it firm. That is the whole logic of working from a proper bill of quantities: every line that can be firm, is.

Reconcile early, reconcile in writing

The moment you know the real cost of a provisional or PC item, tell the client. Not at the next invoice, not at the end of the job. The same day if you can manage it. A two-line message does the work: here is the allowance, here is the actual, here is the difference, do you want to proceed. Clients forgive numbers moving. They do not forgive finding out late.

And adjust downwards with the same enthusiasm you adjust upwards. If the tiles came in at £950 against a £1,200 allowance, the client sees a £250 credit without asking. Do that once and every upward adjustment for the rest of the job is believed. This is the same principle that makes variations and extras painless: transparency before the money, not after.

Make the sums visible in the quote itself

Bury your sums in a paragraph of prose and the client will not register them until they hurt. Put them in their own section, totalled, with a sentence explaining that these are the only figures in the document that can move and exactly how they move. A quote built as a proper itemised bill makes this natural, because provisional and PC items simply appear as flagged lines among the firm ones. That is how QS Quoter lays out its output: a full bill of quantities where every line is visible and editable, so the floating lines stand out against the firm ones instead of hiding among them.

Handled this way, provisional and PC sums stop being a source of suspicion and become the opposite: proof that you price what you can see and refuse to guess at what you cannot. That is what a careful builder looks like on paper.