The budget carries provisional sums nobody has defined

A provisional sum is an honest admission that something isn't designed yet. It stops being honest when the allowance is a rounded guess from an area rate, nobody records what it was assumed to cover, and the budget line quietly hardens into a commitment. When the design lands, the real price differs - it usually differs one way - and the conversation about the gap has no baseline, because nobody wrote down what the sum was for.

The discipline is small: each sum defined - what it covers, what it excludes, what assumptions priced it - with a date by which it must be designed and bought, and a register that shows the drift as it happens rather than at the final account. Undefined sums are where budgets go to move.