Just like in almost any other claim against a professional, after proving that the appraiser owed a duty to the claimant, the claimant must establish the relevant standard of care (i.e. measure for performance) for the appraiser under the circumstances of the particular assignment and that the appraiser breached (fell below) that standard of care. For example, the claimant would need to establish that the appraiser made some error in selecting comparable sales and/or in making valuation adjustments, or made a mistake in measuring the square footage of the subject property.
Generally, the claimant will need to hire an expert witness – a competent appraiser to testify as to what the standard of performance was under the circumstances for the defendant appraiser and that the defendant appraiser violated that standard of care. In doing so, the expert may rely on his/her own experience and knowledge and rules, regulations and other standards, such as those adopted by appraiser professional associations and regulatory agencies, applicable to appraisers.
For example, in Arizona, appraisers must be licensed by the Arizona Board of Appraisal, which has various rules and regulations. Many appraisers are also members of voluntary associations that have adopted rules and guidelines for their members, such as the Appraisal Institute and/or the Appraisal Foundation.