I purchased a multi where the first floor unit was so filled with newspapers, books, boxes of clothes, old take out food containers and personal debris that you could only walk from room to room through small passageways the occupant had cleared. The debris was so heavy that the floor joists were bending and warped and you could only see this from the basement because the floor was not viewable from inside the apartment. The plumbing was all backed up and the occupant was using the sink and buckets for bathroom waste and the rodent and cockroach infestation was unbelievable.
Some of the issues that I found would not have been uncovered during a typical appraisal inspection. I recommend advising the client that extraordinary assumptions that could significantly impact the subject's value will need to be employed due to the lack of availability during the physical inspection. Some clients will balk when they are told an EA is needed. Unless you feel that you can provide an accurate value, encompass all of the extraordinary assumptions that may be needed and are getting adequately compensated for this complex assignment, charge for the trip fee and move on.