Accessory dwelling units
An ADU does not automatically turn a one-unit house into a duplex.
Fannie definition
An ADU is an additional independent living area associated with a one-unit primary dwelling and must provide living, sleeping, cooking and bathroom facilities.
One ADU maximum
Fannie permits only one ADU on the parcel of a one-unit primary dwelling under its general ADU framework. ADUs are not permitted as ADUs on a 2–4 unit property.
Legal status and marketability
Zoning, permitting, market acceptance and appraisal treatment still matter. An illegal second unit cannot simply be relabeled “ADU” to make the property eligible.
Official guide baseline
Fannie Mae Selling Guide · Freddie Mac Seller/Servicer Guide
Property eligibility, valuation method, appraisal findings, AUS messages and lender overlays all matter. The appraisal supports collateral eligibility; it does not replace underwriting.