Thomas D. Horne wrote:

It is the contractors responsibility to plan a compliant
installation. Because a home owner has no way to judge the completeness
of the proposed work the law in most places is that compliance is the
contractors responsibility.

Exactly. If you do this type of work as a contractor, it is your
responsibility to make sure your bid/estimate/quote/whatever covers
all the code requirements. You can't put the burden on the customer,
even if it's just the financial burden ("I have to charge extra
because I neglected to design the job up to code").

Maybe if the customer did not provide adequate information, or
intentionally withheld something the contractor could not learn on
their own, then the contractor could be in the better position. A
smart contractor would choose not to do the job if they suspected
anything like that was happening.