Provisional Sums Explained: What They Mean on a Building Quote

A provisional sum is a figure included in a quote for work or items that can't be fully defined or priced when the quote is written. It is identified as provisional, so both sides know it will be replaced by the actual cost once the work is done or the item is chosen.

It is different from the rest of the quote. A fixed price for defined work means the contractor carries the risk of it costing more than they expected. A provisional sum puts that part of the risk with you: if the work needs more than the allowance, the cost goes up, and if it needs less, it comes down.

You may also see the term prime cost sum, sometimes shortened to PC sum. It is used in a similar way for items to be bought, such as fittings you haven't chosen yet. The allowance covers the cost of the item, and the contractor's fitting is usually priced separately. The same principle applies: the allowance is adjusted to what you actually choose.

A provisional sum is not a contingency the contractor can spend as they like, and it is not a figure they are entitled to in full regardless of the work. It is a placeholder. Our short answer to what a provisional sum is gives the one-paragraph version.

Once the work is done or the item is chosen, the allowance is replaced with the actual cost. How that actual cost is worked out should be agreed before the work starts, not after.

For work. The usual approaches are a rate for the work measured (for example, per length or area), or the time spent at an agreed day rate plus materials at cost or at an agreed markup. Either is fine if it is agreed in advance and recorded.

For items. The allowance is replaced by the price of the item you choose. Ask whether the contractor adds anything for ordering and handling, and if so, how much. It is a legitimate cost, but it should be clear.

Before going over. It is reasonable to ask the contractor to tell you before the cost of a provisional item exceeds the allowance, so you can decide how to proceed rather than learning about it on the final invoice. Agree this at the start and put it in writing.

On the final account. The final invoice should show each provisional sum, the amount it was replaced with and how that amount was arrived at. If the adjustment is a saving, it should come off. Our guide to final payment and snagging covers the end of the job, and invoices and receipts covers what the paperwork should show.

When the uncertain work is uncovered, the contractor should tell you what they have found and what it means for the cost. This is the moment to ask questions: what is needed, why, and what the options are, if there are any.

An adjustment to a provisional sum is not the same as a change to the job. The work was always in the quote; only its cost was uncertain. A change to what you have asked for is a variation, and it is handled differently. Our guide to variations and extra costs explains the distinction.

Keep a note of each adjustment as it happens: what was found, what was agreed and the revised figure. Photographs of the uncovered condition are useful too. It makes the final account straightforward to check, and it avoids any debate about what was agreed. Our answer on what happens if the price goes up covers the wider question.