A provisional sum is an amount of money included in the contract sum to cover work or materials, or both, the extent of which cannot be specifically detailed when entering a contract.  Typically, builders will include a PS for sitework costs. The builder is required to take reasonable steps to ensure an accurate site works cost estimate. However, in some circumstances, unforeseen events mean that the provisional sum is exceeded, sometimes by a significant amount. This can be added to the contract sum.

For example, a building site may seem sandy and clean with minimal siteworks required.  However, there may be large limestone rocks or tree trunks concealed below the surface which only emerge once siteworks commence, causing a considerable cost overrun.