If this is the case, you need to ask the appraiser what his "underwriting guidelines" are in order to try to correct the appraisal under those guidelines. Homes that are of an unusual type often require the appraiser to do a wide area search, and make adjustments based on the locations of those properties, but that doesn't mean that it was done properly. You'll need someone with knowledge of the surrounding communities to make a case, or pay for a secondary appraisal. If the buyer has already given notice that he wants out, release his money and move on! It's unlikely you'll change his feelings.