What ANSI Z765-2021 actually measures

ANSI Z765-2021 is the American National Standard for calculating the square footage of single family houses. It was written for appraisers and real estate professionals, which is why architects tend to encounter it second hand, usually as a line on an appraisal or a listing.

It matters to design work for one reason: it is the only widely accepted definition of what a house’s square footage is. If two people measure the same house and get different numbers, the standard is how you decide who is right.

The rules that change the number

Measurements are taken from the exterior face of the exterior walls. Interior wall thickness is included in the total. That alone explains most of the gap between a tax record and a hand measured interior.

Only finished space counts. Finished means it has a finished floor, wall, and ceiling, and is heated by a permanent system. A finished basement is reported separately from above grade area and never combined into the headline number.

Ceiling height matters. Space under a sloped ceiling counts only where the ceiling is at least five feet high, and at least half of the room must have a ceiling of seven feet or more. Attic conversions live and die on this rule.

Open to below areas, such as a double height living room, are counted once, on the floor where the floor actually is.

What the standard does not do

It does not tell you where a wall is. It is a rule for adding up area, not a drafting standard. A floor plan can be fully ANSI compliant and still be a poor base drawing if the geometry behind it is loose.

That is the distinction Vernier draws between the ANSI floor plans, which are a compliant area statement of the house, and the DWG export, which is the measured geometry you draft over. Both come from the same scan. They answer different questions.