Thanks to all of you for your thoughtful replies. They were very helpful. Ultimately I added a section to my addendum in which I quoted paragraph 1 of the Statement of Assumptions and Limiting Conditions, then stated that I had no information about building permits for the subject, and that obtaining that information was outside of the SOW for this appraisal and was in addition not customary and usual practice among appraisers in this area. Then I made an extraordinary assumption that all work done to the place had been done according to applicable regulations, including obtaining any necessary permits. (I refrained from adding, "thanks for helping me waste my time.")
I expect this to be the end of it, but who knows?