FAQ · 4 min read

Is Square Footage Measured from Inside or Outside?

From the outside. For residential appraisals, gross living area (GLA) is always measured using exterior dimensions. ANSI Z765 -- the national standard used by appraisers -- explicitly requires exterior measurement for this reason: it produces a consistent, reproducible number regardless of who takes the measurement or how the walls are finished on the inside.

Why exterior measurement?

If you measure a room from inside wall to inside wall, you get the livable floor area of that specific room. But if two appraisers measure the same home with slightly different interior tape placements -- or if one includes baseboard thickness and one does not -- they will get different numbers.

Exterior measurement eliminates this ambiguity. You measure each wall of the house from outside corner to outside corner. It is the same measurement every time, regardless of interior wall finishes, thickness, or trim.

How much difference does it make?

Exterior GLA is always larger than interior room totals. The difference equals wall thickness -- typically 4 to 6 inches per exterior wall pair, and less for interior walls (which are not included in exterior measurements but may be measured if you are adding up rooms from inside).

For a typical single-story ranch home with a simple rectangular footprint, the difference between exterior GLA and interior room totals is usually 50 to 150 square feet. For a multi-story home with more complex geometry, the gap can be larger.

This is one reason why a home's listed square footage may appear to differ from what you measure with a tape measure inside.

What about condos and townhouses?

Condos are a notable exception. Condominium square footage is typically measured from the interior -- specifically from the interior face of the unit's perimeter walls. This is because condo owners do not own the exterior walls; they own the air space inside them. ANSI Z765 applies the same standard but acknowledges that local condo association documents may specify a different measurement methodology.

Townhouses that are fee-simple (you own the land and walls) are usually measured from the exterior. Townhouses structured as condominiums follow condo measurement rules.

What about floor plans from CubiCasa, Matterport, or iGUIDE?

Third-party scanning services like CubiCasa, Matterport, and iGUIDE typically measure from the interior using camera-based or LiDAR scanning. Their floor plans show room dimensions based on interior wall faces.

When an appraiser uses one of these floor plans to verify GLA, they need to account for the measurement methodology difference. Interior-based floor plan GLA will be slightly smaller than field-measured exterior GLA. The gap is usually small enough not to cause a discrepancy that matters, but it is worth understanding when comparing a scan-based floor plan to an appraisal report.

When does measurement method actually matter?

• Comparing to MLS listings: MLS square footage is often entered by agents based on tax records, prior appraisals, or manual measurement -- not always ANSI exterior methodology. Discrepancies are common.
• Comparing to tax assessor records: Assessors may use interior measurement, satellite measurement, permit data, or simply the builder's estimate. These often differ from appraisal GLA.
• Before a sale: If the listed square footage is materially wrong, it can affect the appraisal and the deal. Verifying before listing or making an offer is worthwhile.
• Renovation permits: Permit documents may use interior dimensions. These will not match appraiser GLA exactly.

Summary

Appraisers measure from the outside. Interior measurements are always smaller. If your measured room totals do not match a listed square footage, this methodology difference is often the explanation. For ANSI-compliant GLA, exterior measurement is the standard.