How do I resolve a dispute over property valuation ?

Property appraisals are usually based upon the "highest and best use".  If the property could be used for a more valuable purpose if permits were obtained, then the appraiser can note that, but then must also deduct the cost for obtaining those permits, and the permits must be something reasonable.  The use must be something that is allowed in the zone.  An appraisal is just an "opinion" of value and one person's opinion could be different from another person's opinion.