4.12. Appraisers’ Use of Supporting Experts’ Opinions.Some appraisal assignments may require the appraiser to rely on other experts’ opinions on technical or other specialized issues.1178 “An expert cannot be an expert in all fields, and it is reasonable to expect that experts will rely on the opinion of experts in other fields as background material for arriving at an opinion.” 1179 But the appraiser cannot merely assume such supporting experts’ reports are accurate and reliable. Rather, the appraiser must review all supporting opinions and can rely on or adopt them only if the appraiser determines, after review, that all supporting opinions are credible, reliable, and factually supported.1180 As the Tenth Circuit succinctly stated: “[Any expert] opinion . . . must be founded upon substantial data, not mere conjecture, speculation or unwarranted assumption. It must have a rational foundation.”1181 An appraiser who fails to ensure the rational foundation of all supporting opinions and other underlying assumptions will be left with an “ultimate opinion of value [that] is virtually devoid of factual moorings, depriving it of virtually any evidentiary value.”1182 

The appraiser must carefully analyze subsidiary experts’ reports to ensure a full understanding of the bases for their findings and the impact on the appraisal. The precise steps necessary to ensure the reliability of a particular subsidiary expert’s opinion will of course depend on the subject matter. Broadly, however, important considerations for the appraiser include: 

• Are subsidiary experts’ opinions confirmed by market studies or other appropriate analyses? 

• Did subsidiary experts credibly reconcile data or other facts that may contradict their conclusions? 

• Did subsidiary experts gather relevant data in a methodical manner? 

• Are the assumptions underlying subsidiary experts’ opinions reasonable and appropriate? 

• Are subsidiary experts’ conclusions substantiated by other evidence? 

Appraisers’ reliance on subsidiary expert opinions that failed to adequately address these concerns has resulted in the rejection of all valuation evidence based on such unsupported opinions—including appraisers’ conclusions of highest and best use, methodology, and ultimate opinions of value.1183 However, with appropriate verification of reliability and support (as recognized in a case affirmed by the Eleventh Circuit), “relying on a sub-consultant’s report is a common, respected, and approved occurrence in appraisal practice and[ ] ‘to hold otherwise would effectively demand an inconceivably broad area of expertise from any appraiser.’”1184 

Unit Rule Considerations.The results of subsidiary valuation reports, such as mineral, fixture, or timber valuations, cannot simply be added to the value of the land to arrive at a value of the property as a whole without proper analysis by the appraiser. To do so would violate the unit rule (discussed in Section 4.2.2) and professional appraisal standards.1185 These components are to be considered, but only in light of how they contribute to the market value of the property as a whole.1186