The problem with this advice is it means the appraiser is turning in a misleading report on the rationale the client asked for it. The client is not signing the appraisal, we are. That is why the option exists for an appraiser to refuse an assignment condition and thus decline the order (or make the client remove the condition). Saying the part built ADU exists but we excluded it at client's request does not erase the problem.
Okay, a client asked the appraiser to exclude the part finished ADU. Clients can ask for anything, Seems they often find an inexperienced appraiser to hand these types of assignments off to, perhaps knowing an experienced one would not go along with it. The market value opinion purpose asks for to value a subject property as a whole., not a segment of it. That is why we don't get to exclude pools, garages, or half a house because a client asks for it on a URAR. (unless it meets a pre-printed HC, which normally it does not )