For detached single family homes, most appraisers use Gross Living Area (GLA) as described by FNMA and FHLMC in their Selling Guide B4-1.  This is similar to the ANSI standard for measuring (Z765) but allows less precise rounding and departure from the standard if required for consistency.  Reasons for departure from this standard may include situations where or where local builders, public records or sales people use a different standard. These situations are rare and are most commonly an issue with split-level homes, hillside homes, or homes that have large two-story ceiling areas and/or foyers.  A diligent appraiser is always on the lookout for inconsistencies between brokers’ estimates, public records, and builders’ estimates that may not be measured to the same standard and to ensure that these variations do not affect the reliability of the appraisal.  In the area we services tax assessment offices can be inconsistent with their measuring standards as they do not always have recent information, do not have the opportunity to view the inside of homes, or used poorly trained or inexperienced assistants.  They often measure second-floor area, garage area, basement area, or porches incorrectly or guess at their size.