Provisional sum

A provisional sum is the contractual mechanism for tendering around a hole in the definition. The owner's side inserts a fixed figure for the undefined element — ground works pending investigation, a specialist package not yet designed — so all bidders carry the same placeholder and the tenders remain comparable. When the work is eventually defined and instructed, the sum is omitted and the real valued cost substituted, with the contract price adjusted either way.

Standard forms distinguish defined provisional sums — where enough is known for the contractor to have allowed for them in programme and preliminaries — from undefined ones, where the contractor could not, and any time consequence becomes the owner's risk. The distinction decides who pays for the disruption, not just the work.

The pathology is overuse. A tender heavy with provisional sums is a lump-sum contract in name only: price certainty has been deferred, not achieved, and each sum matures into a negotiation. Provisional sums measure, quite precisely, how unfinished the design was when it went to market.