A provisional sum is an estimate of the cost of completing a particular job. A provisional sum includes the cost of supplying any materials needed for work.
If, after making all reasonable inquiries, a builder cannot provide a definite price for an item at the time the contract is signed, the builder should provide a provisional sum. Determining the provisional sum allows work to begin with the expectation that, as the work continues, the amount may change based upon the scope of the work. The client is responsible for paying the difference between the provisional sum and the actual cost.