I think CASP certification and renewal is expensive and unnecessary for a local building dept.
Application fee - $500
Examination fee -$800
Certification fee -$300
Total initial certification=$1,600.
But also renewal, that is another $500 every three years.
That does not include the cost of training courses, which I a believe are more important than the certification.
Who would be your CASp? Your BO?
What advantages does it have? The knowledge of the studying and training is the important thing.
The certification is no more meaningful then being LEED-AP.
Although your question is focused on liability, which is specifically addressed on the states website listing certified individuals:
"Statement: Notwithstanding any other provision of law, a state agency or employee of a state agency may not be held liable for any injury or damages resulting from any service provided by a certified access specialist whose name appears on this list published pursuant to Government Code section 4459.7(a)."
So it couldn't hurt. Worth considering just to help focus on studying the laws and codes and staying current.