Learn · Square Footage Basics · 5 min read

Part of: How to Measure Square Footage: The Complete Guide

Does Square Footage Include Walls?

It depends on which measurement standard is being used. Appraisers measure from the exterior of the building, so wall thickness is included in the square footage figure. Interior room-by-room measurements exclude wall thickness. This difference is small per wall, but it adds up — and it explains why different sources often report different square footage for the same home.

The appraisal standard: exterior measurement includes walls

Under ANSI Z765-2021, the standard used by appraisers for residential properties, gross living area (GLA) is calculated using exterior dimensions. An appraiser walks the perimeter of the home and measures each exterior wall from outside corner to outside corner.

Because the measurement is taken at the exterior face of the wall, the wall thickness itself is included in the floor area calculation. A 40-foot exterior wall dimension includes 6 to 8 inches of exterior wall depth on each end — space that is occupied by framing, insulation, and siding, not usable living area.

This is the intentional design of the standard. Exterior measurement is faster, more consistent, and easier to verify from outside the building. It also avoids the complexity of accounting for interior wall thickness, which varies by wall type (exterior walls, load-bearing interior walls, partition walls) and by construction method.

Interior measurement: walls are not included

When you measure a room by walking inside it and measuring wall-to-wall, you are measuring the interior clear dimension. The wall thickness is on the other side of the surface you are measuring to. Interior room measurements therefore exclude wall material.

This is what most people intuitively think of as "room size" — the usable floor space you can walk on and furnish. It is also what phone-based measurement apps typically produce, because they measure the distance between the interior wall surfaces you point them at.

When you add up all the interior room measurements in a home, the total will be less than the appraiser's exterior-measured GLA for the same home. The difference is the aggregate wall thickness across all exterior walls.

How much difference does wall thickness make?

Wall thickness varies by type:

• Exterior walls (wood frame): 5.5 to 7.5 inches total (2×6 framing with sheathing and siding)
• Exterior walls (brick veneer): 8 to 12 inches
• Interior load-bearing walls: 4.5 to 5.5 inches (2×4 framing with drywall both sides)
• Interior partition walls: 4.5 inches (2×4 framing with drywall both sides)

For a typical single-story 2,000 sq ft home with a perimeter of roughly 180 linear feet, exterior walls averaging 6 inches thick contribute approximately:

180 linear feet × 0.5 ft wall depth ≈ 90 sq ft

In practice, the difference between exterior-measured GLA and the sum of interior room dimensions typically runs 50 to 150 sq ft for a typical single-family home, depending on size, shape complexity, and wall construction type. For a 2,000 sq ft home, that is roughly 3 to 8% of total area.

Why this causes measurement discrepancies

This is one of the primary reasons why different sources report different square footage for the same home:

• Phone measurement apps (AR, LiDAR) measure interior dimensions. A phone-measured floor plan will systematically understate GLA compared to an appraiser's exterior measurement.
• Hand-drawn floor plans based on interior room measurements will also understate GLA for the same reason.
• Floor plan apps like CubiCasa that scan interior spaces produce interior dimensions, then may or may not add estimated wall thickness to generate an exterior GLA figure. Check the documentation for any tool you are using.
• County assessor records sometimes use interior measurements, sometimes exterior, and sometimes use a hybrid that creates its own inconsistencies.

When you are comparing a self-measured figure to an appraiser's GLA, expect the appraiser's number to be higher if your measurement was interior-based. This is not an error — it is the predictable result of different measurement conventions.

Do interior walls between rooms count?

In exterior-measured GLA, interior walls are implicitly included because the measurement captures the full exterior footprint regardless of interior layout. A 4-inch interior wall between a bedroom and a hallway occupies 4 inches of the floor plan's total width, and that 4 inches is counted in the exterior dimension.

In interior room-by-room measurement, each room is measured to the interior face of each bounding wall. The wall thickness between rooms is not counted in either adjacent room's measurement, so all interior wall material is excluded.

For a home with many interior partitions (lots of small rooms), this difference can be more significant than for an open-plan home with few interior walls. An open great room has only exterior walls to account for. A compartmentalized floor plan with many rooms has both exterior wall thickness and interior wall thickness excluded from the interior measurement total.

Which number should you use?

For real estate transactions, lending, and appraisal purposes: use exterior-measured GLA per ANSI Z765. This is the number that appears on appraisal reports, that lenders rely on, and that comparable sales in the MLS are based on (when measured correctly).

For furniture layout, renovation planning, and personal reference: interior dimensions are often more useful because they represent the space you can actually occupy and furnish.

For price-per-square-foot comparisons: make sure all homes in your comparison use the same measurement convention. Mixing exterior-measured GLA with interior-measured room totals will produce misleading ratios.

How PlanSnapper handles this

When you upload a to-scale floor plan to PlanSnapper and trace the exterior perimeter, you are measuring the exterior footprint — the same approach appraisers use. The traced area includes wall thickness, producing a GLA figure consistent with ANSI Z765 methodology.

If your floor plan shows only interior room dimensions (a room-by-room layout without explicit exterior wall indication), the traced perimeter will be slightly smaller than the true exterior GLA. For most practical purposes this is acceptable; for a formal GLA verification you would want to either add wall thickness or use a floor plan that includes exterior wall representation.

Related: ANSI Z765 Square Footage Standard · How Appraisers Calculate Square Footage · How to Measure Square Footage with Your Phone · What Counts as Square Footage in a House? · FAQ: Exterior vs Interior Square Footage Measurement

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