A provisional sum is a placeholder with a number on it. The works include something whose scope is not yet settled, such as a borehole, a lift, a kitchen fit-out or a drainage connection, so the bill carries an allowance rather than a measured item.
Two things follow from that, and both surprise people the first time.
It is not a price
A provisional sum is expected to be adjusted. When the real scope is known, the sum is omitted and the actual measured work is substituted. If the allowance was KES 800,000 and the work comes in at KES 1.3 million, the contract sum goes up by the difference. Nobody has done anything wrong. That is the mechanism working exactly as designed.
The confusion arises because a contract sum containing several provisional sums looks like a firm number and is not. It is a firm number plus a set of open questions, each carrying a guess.
It is where budgets quietly leave
A build with generous provisional sums has a contract sum that reads as settled and behaves as provisional. Two disciplines keep that under control.
Convert them early. As soon as the information exists to specify the work, specify it, get it measured and priced, and replace the allowance with a real figure. Every provisional sum still open in month six is a number that can still move.
Track the adjustments as their own total. Not folded into the general noise of the project. A single running line reading "provisional sums: allowed KES 3.1m, adjusted to date KES 3.9m, three still open" tells an owner more about where the budget stands than any other single figure.
Prime cost sums, which are not the same thing
A prime cost sum, or PC sum, works similarly but covers goods supplied by a nominated supplier rather than undefined work. The allowance is for the item itself, such as sanitary ware, ironmongery, or a particular tile, with the contractor's profit and attendance priced separately against it.
The practical difference: a provisional sum covers work whose extent is unknown, and a prime cost sum covers an item whose choice is unmade. Both adjust. A PC sum adjusts when you finally choose the tiles, and the adjustment is the difference between the allowance and what you chose, which is why the tile showroom is a budget event and not a decorating one.
A worked example
The bill carries a provisional sum of KES 450,000 for the drainage connection, because nobody yet knows how far the connection runs or what it crosses.
The survey comes back. The run is longer than assumed and crosses a services easement that requires hand excavation. The measured work prices at KES 720,000.
The sum of KES 450,000 is omitted. The measured work of KES 720,000 is added. The contract sum rises by KES 270,000, and that KES 270,000 belongs on the running adjustments total on the day it is agreed, not at the final account.
What goes wrong
They are read as prices. An owner budgets to the contract sum, three provisional sums adjust upward, and the overrun is experienced as a surprise although it was structurally certain from the start.
They are left open too long. Information that could have been obtained in month one is obtained in month seven, by which point the work is on the critical path and there is no time to price it competitively.
They are used to hide an incomplete design. A bill with a dozen provisional sums is not really a measured bill. It is a partial measure with a list of unanswered questions, and it should be recognised as one before it is treated as a budget.