If you disagree with the appraiser’s opinion of value, you may challenge the value. For private appraisals, simply contact the appraiser.
For appraisals for lending purposes, contact the lender for advice on how to submit a reconsideration of value. The GSE’s have newer procedures for borrower-initiated reconsiderations of value.
For the reconsideration of value, you will want to identify any factual errors identified in the appraisal, along with sales the appraiser did not consider, that you believe are likely substitutes for the subject property.