Zoning

Zoning is the set of local rules that divides land into districts and controls how each one can be used, such as for homes, shops, or industry. Zoning rules also limit how big buildings can be, how tall, and how close to the lot lines.

Some of those limits are written in terms of floor area, through measures such as floor area ratio or a maximum house size. Each rule defines floor area its own way, and those definitions often differ from gross living area.

Zoning does not change how an appraiser measures a house. Gross living area follows ANSI Z765 in every part of the country, whatever the local zoning allows. Whether a space is permitted is a separate question, handled outside the measurement.

Further reading: Zoning on Wikipedia.