Answered: NEC ECC: Use of provisional sums as a Z clause?

Frankly no I don't like to see provisional sums in NEC contracts as the machinery of the contract is really not set up to deal with it properly. If approx. 25% of the work is so ill defined that it needs a provisional sum then this probably shouldn't be under an Option B contract at all.

I suspect the above will just lead to problems. At best the contract will be ignored and at worst it will end in dispute.