What is an extraordinary assumption in appraisal?

An extraordinary assumption in appraisal refers to a fact or premise that is assumed to be true for the evaluation but could significantly change the appraiser's conclusions if it is found to be false. This type of assumption is notable because it involves a situation or condition that is not typical or regular within the market. Appraisers often make extraordinary assumptions to simplify the analysis or when specific information is not available. However, they must identify these assumptions clearly in their reports and understand the risks involved, as any negative impact from these assumptions could lead to inaccurate property valuations.

The other options do not accurately describe an extraordinary assumption. A widely accepted market trend reflects general market conditions rather than specific assumptions. An assumption based on vendor reports does not capture the potential implications of its truthfulness. A common understanding in the industry is a general consensus but does not indicate uncertainty or potential impact on conclusions like an extraordinary assumption does.