Provisional sum

In practice for trades

A provisional sum keeps an unknown from wrecking a price. You cannot see behind the wall, under the floor or inside the ground, so instead of guessing high and losing the job or guessing low and eating the difference, you state an allowance and adjust it later.

Two kinds turn up. A defined provisional sum is work you know is coming but cannot measure precisely, so the contractor is expected to have allowed for its effect on programme and preliminaries. An undefined provisional sum is work with too little information to plan around at all, and the contractor is generally entitled to time and prelims consequences when it lands. The distinction decides who carries the knock-on cost, not just the work itself.

Two habits keep them out of trouble. Say on the quote exactly what the allowance covers, so a later argument is about scope rather than memory. Agree the real figure in writing before the work is done, not when you invoice: a provisional sum is an allowance to be adjusted, never a blank cheque.

Worked example

A rewire quote includes a £1,500 provisional sum for the cellar, where the existing wiring cannot be inspected until the boards come up. Once opened, the work is measured properly at £2,150. The provisional sum is omitted from the account and the real figure substituted, adding £650 to the final price.