What is a provisional sum?
A provisional sum is an estimated amount included in a building contract for items or works whose final cost cannot be fixed at the time of quoting, such as excavation depth or selected finishes.
When a custom home builder prepares a quote, not every cost can be nailed down before construction starts. A provisional sum is a placeholder figure inserted into the contract for work or materials where the exact scope or price remains uncertain. This allows the quote to move forward without guesswork holding up the process.
Provisional sums commonly cover:
- Excavation and site preparation, where soil conditions are not yet fully known
- Fixtures and finishes the client has not yet selected (bathroom tapware, light fittings, kitchen cabinetry)
- Allowances for items ordered by the client that carry variable lead times or pricing
- Contingencies for site-specific requirements like drainage work or rock removal
The provisional sum sits in the contract as a fixed line, but both builder and client understand it will be adjusted to reflect the actual cost when details are locked in. This prevents disputes later: the builder is not guessing, and the client knows the real price will be confirmed before the work happens. When the item is finalized, the provisional sum is replaced with the actual invoice or variation to the contract.
Builders working with custom home design and build use provisional sums to manage the gap between detailed design and site reality, keeping quotes honest and scope clear from the start.