"Pat" wrote
So what you are saying is that the law says you
cannot rely at all on the old drawings and then confirm the details,
and check for code compliance as part of stamping plans for the new
building and the reno.

Structurally, I would never rely on *unknown* drawings, and I would never
include someone else's drawing with mine.
The only exception are standard detail sheets like I mentioned before.
I would design the new part and do so so that it was completely independent,
structurally, from the existing building and connect the two only
aesthetically.
You see, just because the existing drawings indicate there is a proper
footing, perimeter beam, etc., where they are supposed to be there is no
guarantee those things were actually installed so thus, my new addition
could fail due a lapse of judgement on my part in that regard.
So, I don't do it.....for any amount of money.
You can't but a good reputation but you can throw one away real cheap.