Measurement Standards · 4 min read

Exterior vs Interior Square Footage Measurement: Which Method Applies?

Not all square footage is measured the same way. The method depends on the property type. Most homes are measured from the outside. Condos, co-ops, and TIC units are measured from the inside. Here is why — and what it means when you use PlanSnapper.

The standard rule: most properties are measured from the exterior

For single-family homes, townhouses, duplexes, and most attached housing, square footage is measured from the exterior face of the above-grade walls. This is the method required by the ANSI Z765 standard and required by Fannie Mae, Freddie Mac, FHA, and VA for appraisals on those property types.

Exterior measurement includes the wall thickness itself as part of the square footage. A typical wood-framed wall adds 4 to 6 inches per side. On a 40-foot wall measured from both sides, that is 8 to 10 inches of wall thickness included in the total — which across a whole house can easily add 40 to 80 square feet compared to measuring only the interior space.

The exception: condos, co-ops, and TIC units are measured from the interior

Condominiums, cooperative units, and tenant-in-common (TIC) units follow a different convention: square footage is measured from the interior face of the surrounding walls, typically drywall face to drywall face.

The reason is ownership. In a condo or co-op, you do not own the exterior walls — the building structure belongs to the HOA or cooperative corporation. What you own is the interior airspace and the finishes. Measuring from the exterior would include space you do not legally own.

TIC units follow the same logic. When multiple parties own undivided interests in a property with exclusive occupancy rights to specific units, the measured area reflects the interior living space, not the structural shell.

Why this matters when comparing properties

A 1,200 square foot condo measured from the interior and a 1,200 square foot house measured from the exterior are not the same amount of livable space. The house is actually somewhat smaller inside because its measurement includes wall thickness that a condo measurement does not.

This is one reason why price-per-square-foot comparisons between condos and houses are not apples-to-apples. It also explains why a condo's square footage sometimes looks more generous relative to its floorplate than a comparable single-family home.

How to handle this in PlanSnapper

PlanSnapper measures whatever perimeter you define. The result reflects the floor plan you upload and the boundary you approve.

For single-family homes, townhouses, and most attached housing: use a floor plan that shows exterior dimensions, such as an appraisal sketch, a CubiCasa or Matterport floor plan, or an architect's site plan. Approve the exterior wall perimeter. PlanSnapper will return an exterior measurement consistent with ANSI Z765.

For condos, co-ops, and TIC units: use a floor plan that shows interior dimensions, such as a unit floor plan from the developer, a condo association drawing, or a plan that clearly shows wall-to-wall interior space. Approve the interior perimeter. PlanSnapper will return an interior measurement consistent with condo measurement convention.

If you are unsure which floor plan type you have, check whether dimensions on the plan align with interior room sizes or with overall building footprint. An interior plan will show individual rooms adding up to the total. An exterior plan will show a slightly larger overall footprint.

Quick reference by property type

• Single-family home: exterior measurement
• Townhouse / rowhouse: exterior measurement
• Duplex / triplex / fourplex: exterior measurement per unit
• Condominium: interior measurement
• Cooperative (co-op): interior measurement
• Tenant-in-common (TIC) unit: interior measurement
• Manufactured home: exterior measurement (per HUD standards)