Provisional sums and PC sums: pricing what you can't see yet

↑ Part of the Quoting series · How to quote a job properly

Some of a job can't be priced on the day you quote. The joists are under a floor that hasn't come up, or the customer hasn't picked the tiles. You can guess and hope, you can refuse to give a fixed price at all, or you can put a labelled allowance on the quote for just that part. The labelled allowance is the honest one, and it comes in two kinds. A provisional sum is an allowance for work you can't define yet. A prime cost sum, or PC sum, is an allowance for goods the customer hasn't chosen yet. Either way the rest of the quote stays a fixed price.

This page sits under our How to Quote a Job Properly guide. It covers the difference between the two, how to word them so nobody argues later, and how to settle them when the real figure turns up.

Part 1Why not just guess

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A guess has two ways to go wrong, and both cost you. Guess low and you either swallow the difference or have an awkward conversation about a price the customer thought was fixed. Guess high to be safe and the whole quote looks expensive next to a trade who left the item out entirely.

A labelled allowance avoids both. The customer can see that one line may move and why, and every other line is a price you have committed to. That is a stronger position than calling the whole job an estimate, which tells the customer nothing about which parts are firm. Our page on quote vs estimate explains why that difference matters.

Part 2Provisional sums: work you can't define yet

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Use a provisional sum when you know some work will probably be needed but cannot see enough of it to price it. The typical cases:

• Hidden condition. Rot in the joists, the state of a lintel, what is behind old plaster.
• Work that depends on someone else. A building control requirement that will not be known until the inspection, or a structural engineer's detail still to come.
• Ground you cannot see, such as the foundations of an extension before the trench is dug.
• Unknown extent. Repointing "as required" on an elevation you have only seen from the ground.

Write it so it cannot be misread. Say what the allowance is for, how much it is, and how it will be settled:

On the quote: “Provisional sum of £400 for repairs to floor joists, if required once the floor is lifted. Labour charged at £365 a day and materials at cost plus 15%, or omitted if not needed.”

The last clause does the work. It tells the customer the sum is not a fee you collect regardless. If the joists are sound, the £400 comes off the final bill. If they are worse than allowed, the extra is charged on the basis already written down, and nobody negotiates a rate in the middle of the job.

On larger contracts you may see provisional sums described as defined or undefined. A defined one comes with enough detail (what the work is, roughly how much, where it sits in the programme) that the contractor is expected to have allowed for its time and overheads in the price. An undefined one does not, so the contractor's time and overheads for it are paid on top. On a domestic job, you rarely need the jargon. Just say in plain words what the sum covers and how the final figure will be worked out.

Part 3PC sums: goods the customer hasn't chosen

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Use a PC sum when you know the work but not the item. The customer wants tiles but has not picked them; there will be a basin, a kitchen tap, a front door, a set of light fittings. You price the fitting as normal and put an allowance in for the item itself.

On the quote: “PC sum of £30 per m² for wall tiles, supply only. Final cost adjusted to the tiles chosen, with anything over the allowance charged at cost plus 15%.”

Three things to get right:

• Say whether the allowance is per unit or a total. "£30 per m²" and "£600 for tiles" behave differently when the area changes.
• Say whether it is supply only. The fitting is in your price; the PC sum is only the goods. Mixing the two is how a cheap allowance turns into an argument.
• Say how the difference is charged. Most trades add their usual markup to anything over the allowance, because buying, collecting and handling a more expensive item costs them more too. If you do, write it down.

PC sum as quoted, 20 m² at £30	£600.00
Tiles chosen cost £45 per m², the extra over the allowance	£300.00
Your markup on the extra, at 15%	£45.00
The tile line on the final account	£945.00

The customer chose a more expensive tile, and the quote already said how that would be charged. Nobody had to renegotiate, and the rest of the job's price did not move. Our guide to pricing a bathroom job shows where an allowance like this sits in a whole job.

Part 4Keeping allowances honest

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An allowance is only fair if it is realistic. A PC sum of £10 per m² for tiles nobody sells at that price makes your quote look cheap and sets up a disappointment later. Customers remember the number on the quote, not the word "provisional" beside it. Price the allowance at what a sensible version of the item actually costs.

Keep the number of allowances small, too. One or two on a job reads as honesty about the unknowns. Eight reads as a quote with no fixed price in it at all, and the customer would be right to treat it as an estimate.

Part 5Settling them at the end

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When the real figure is known, settle each allowance in writing:

• Record the actual cost with the receipt or the time sheet behind it.
• Show the adjustment against the allowance, up or down, on the final invoice or the final account.
• Treat anything genuinely new as a variation. If lifting the floor turns up something the provisional sum was never meant to cover, that is a change to the job, priced and agreed before you do it. Our guide to handling variations covers how.

A customer who sees £400 allowed, £260 spent and £140 taken off the bill trusts the rest of your figures more, not less.

Common questionsProvisional sums and PC sums: quick FAQ

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What is the difference between a provisional sum and a PC sum?

A provisional sum is an allowance for work you can't define yet, such as repairs you can't see until something is opened up. A PC (prime cost) sum is an allowance for goods the customer hasn't chosen yet, such as tiles or a basin. The fitting of PC goods is normally in your fixed price.

Does a provisional sum make my quote an estimate?

No. Only the labelled line can move, and the quote says how. Every other line stays a fixed price, which is what makes it a quote.

Can I charge a markup on a PC sum?

Yes, if the quote says so. Most trades add their usual markup to anything spent over the allowance. Write the percentage on the quote so it is agreed before the customer chooses.

What if the work under a provisional sum isn't needed?

Then the sum comes off the final bill. A provisional sum is an allowance, not a fee, and saying so on the quote is what makes a customer comfortable accepting it.

What if the real cost is far more than the allowance?

Tell the customer as soon as you know, before you carry on. If the extra is within what the sum was meant to cover, charge it on the basis the quote set. If it is something new, treat it as a variation and agree it in writing first.

This guide is general information for UK tradespeople, current to the best of our knowledge in 2026, and is not tax, accounting or legal advice. Rates, thresholds and how they apply to you change, so confirm your position with HMRC, a qualified accountant or a solicitor before relying on it. TradeDraft accepts no liability for decisions made on the basis of this guide. Questions: hello@tradedraft.co.uk.