Provisional Sum

An estimated allowance included in a quotation for work that cannot be accurately priced in advance, covering both materials and labour.

A provisional sum is an estimated budget allowance included in a contractor’s quotation for work that cannot be precisely measured or priced until it is underway. Unlike a PC sum, which covers just the cost of a specific product, a provisional sum covers the full cost of a piece of work — materials and labour combined.

When provisional sums are used

Provisional sums appear when there is genuine uncertainty about what work will be needed. Common examples:

  • Groundwork and drainage — the exact condition of underground pipes or foundations is unknown until excavation begins
  • Structural repairs — the extent of rot, damp, or structural damage hidden behind walls or under floors
  • Asbestos removal — the quantity and location of asbestos-containing materials may not be fully known until the strip out is complete
  • Service upgrades — whether the existing electrical consumer unit or boiler can support the new layout, or needs replacing

How provisional sums are settled

Once the work is complete, the provisional sum is replaced by the actual cost:

  1. The contractor records the materials used and time spent
  2. The actual cost is compared to the provisional sum
  3. If the actual cost is lower, you pay less
  4. If the actual cost is higher, you pay more (this should be flagged as a variation for your approval)

Protecting yourself

Provisional sums carry risk because the final cost is uncertain. To manage this:

  • Ask your contractor to explain the basis for each provisional sum — what scenario are they estimating for?
  • Request that any provisional sum work is approved by you before it proceeds, especially if it will exceed the allowance
  • Keep a contingency budget of 10-15% on top of your total quote to absorb provisional sum overruns
  • Where possible, investigate uncertain areas before signing the contract (e.g., a drain survey, damp inspection, or asbestos test) to convert provisional sums into firm prices

The fewer provisional sums in your scope of work, the more predictable your final cost will be.