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New Construction GLA Discrepancies and the Proper Use of Extraordinary Assumptions

This white paper is intended to provide Appraisify appraisers and reviewers with clear, USPAP-compliant guidance for handling Gross Living Area (GLA) discrepancies in new construction and model home assignments, particularly when tax assessor data, third‑party aggregators, and builder‑provided specifications conflict.

 
The goal is to:
  • Establish a consistent, defensible approach
  • Reduce friction between appraisers, QC, and clients
  • Protect appraisers through proper USPAP disclosures
  • Align expectations for lender and investor clients
1. Why GLA Conflicts Are Common in New ConstructionUnlike existing housing stock, new construction properties often exist in multiple data states at the same time:
  • Preliminary permit records (base model)
  • Partially updated tax assessor records
  • Public‑record aggregators (Zillow, Redfin, etc.)
  • Builder plans and specifications (as‑built intent)
These sources frequently do not match, and none—except builder documentation—necessarily reflect the final configuration at or near completion.Key Reality
County assessor data is often estimated, provisional, or outdated for new construction until final inspections occur and the next update cycle is completed.
It is therefore not unusual to see:
  • Multiple GLA figures for the same property
  • Placeholder square footage based on base plans
  • Tax cards updated months after actual completion
This is a data‑timing issue, not an appraiser error.2. Limitations of Tax Assessor Data for New HomesCounty appraisal districts typically rely on:
  • Permit filings
  • Field inspections
  • Periodic reappraisal cycles
For new construction, this means:
  • Square footage may be estimated at the time of record creation
  • Structural options and upgrades may not yet be reflected
  • Model homes may change configuration after initial filings
Counties themselves provide formal processes to correct square footage, which implicitly acknowledges that errors and omissions occur.Practical Implication for AppraisersTax assessor data should not be treated as authoritative for GLA in new construction assignments when more reliable, contemporaneous sources exist.3. Why Zillow and Similar Aggregators Are Not Reliable HerePublic‑facing platforms such as Zillow and Redfin:
  • Rely heavily on public record feeds
  • Apply automated data normalization
  • Often default to base model assumptions
For new construction, these platforms:
  • May show different GLA figures than the county
  • Rarely reflect structural upgrades
  • Lag actual construction progress
Appraisal GuidanceThese sources may be used for general market context only, not for definitive subject characteristics in new construction.4. Builder‑Provided Specifications as the Best Available SourceWhen available, builder‑provided plans, specs, and feature sheets represent the most reliable depiction of:
  • Intended GLA
  • Structural options
  • Finished configuration
For model homes and pre‑completion assignments, builder documentation is often the only source that reflects the property as it will exist at completion.This is particularly true where:
  • The home is not yet complete
  • Final CO has not been issued
  • Tax records are clearly inconsistent or preliminary
5. USPAP Framework: Extraordinary AssumptionsDefinition (USPAP)An Extraordinary Assumption is an assumption that:
  • Is presumed to be true for the purpose of analysis
  • Cannot be fully verified as of the effective date
  • If found to be false, could affect the appraiser’s opinions or conclusions
USPAP explicitly allows extraordinary assumptions when:
  • They are reasonable
  • They are clearly disclosed
  • The appraisal remains credible for its intended use
6. Why Extraordinary Assumptions Are Appropriate for New Construction GLAUsing an extraordinary assumption allows the appraiser to:
  • Rely on builder‑provided GLA when tax data is unreliable
  • Avoid mischaracterizing the subject based on placeholder data
  • Clearly disclose the conditional nature of the information
  • Remain USPAP‑compliant and defensible
This is not a shortcutIt is a recognized and expected tool for:
  • New construction
  • Model homes
  • Pre‑completion valuations
7. Recommended Extraordinary Assumption Language
“This appraisal is developed under the extraordinary assumption that the subject property will be completed in accordance with the builder‑provided plans and specifications, including the reported gross living area. If this assumption is found to be false, the opinions and conclusions contained herein may be affected.”
This language:
  • Properly discloses risk
  • Protects the appraiser
  • Aligns with lender expectations
8. Appraisify Policy PositionFor new construction and model home assignments:
  • Builder documentation may be relied upon for GLA when clearly disclosed
  • Tax assessor and aggregator data may be supplemental only
  • GLA conflicts should be resolved using extraordinary assumptions, not forced conformity to unreliable sources
Appraisers are encouraged to:
  • Explain data conflicts in the report
  • Cite builder documentation as the primary source
  • Avoid representing provisional data as definitive
9. Key Takeaways
  • GLA discrepancies are common in new construction
  • County tax records are often provisional or outdated
  • Zillow and similar platforms are not authoritative for as‑built specs
  • Builder plans are typically the best available source
  • Extraordinary assumptions provide USPAP‑compliant protection
10. Final Note
This guidance is designed to support appraisers, reduce unnecessary revisions, and ensure consistent, defensible outcomes across Appraisify assignments. When in doubt, clear disclosure and proper extraordinary assumptions are always preferred over reliance on unreliable data sources.