A homeowner is permitted to challenge the result of an appraisal.  You can submit a request for reconsideration of value if you have 3 recent comparable sales that you feel are more comparable than the ones used by the appraiser.  The appraiser is required to review those comparable sales presented and either utilize them or address why they cannot be used as a comparable.  If there are no additional comparable sales, there is no recourse for a simple disagreement with the appraisal as the guidelines do not permit you to obtain another appraisal until the initial appraisal expires.  Once the appraisal expires, you can start the loan application again and obtain another appraisal at that time.