I suggest reading your contract's general conditions to see what is included in the definition of "overhead" for changes.
If the BIM work is only for contractor's convenience, it would seem to be their cost, not to pass on to you.
If you require that they turn over BIM documentation at closeout as a form of as-builts, there might be a leg for them to stand on.
Then, talk to whoever is responsible for editing your general conditions and come up with something that is reasonable and fair, for future projects.
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Joel Niemi AIA
Joel Niemi Architect
Snohomish, WA
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