Many extra costs come from instructions you did not know about. Your architect, engineer or manager may have given them.
How it flows
Set a rule that only one person can instruct the contractor. All other requests go through that person.
Log each instruction with its date, source and the work it affects.
Check the log against the contractor's claims. Any claim without an instruction needs a question.
Ask for a copy of every instruction, or use an independent register that logs each instruction sent to the contractor, by whom and when.
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