Does an ADU count as square footage?
Published August 27, 2026
Detached, no. Attached, sometimes. Under ANSI Z765 a detached accessory dwelling unit is a separate structure, measured and reported on its own and never added to the main house's gross living area. An attached ADU (a converted wing or basement unit) can count only if it is finished, above grade, and connected to the house through finished space; a unit with its own outside entry and no interior connection is reported separately.
How appraisers report an ADU
Fannie Mae asks for the ADU's square footage, room count, and rental income on their own lines, and requires the appraiser to say whether it is a legal unit. A basement ADU is below-grade area regardless. The ADU is valued as an accessory unit, which in many markets is a large adjustment, separate from the GLA math.
Why the GLA line matters less than people think
Buyers looking for an ADU pay for the unit, not for 600 extra square feet of GLA. Keeping it separate lets the appraiser compare the house to houses and the unit to units. A detached guest house or garage apartment is handled the same way.