Which rent can count?
Possibly. Fannie Mae allows qualifying rent from one ADU on a one-unit principal residence for eligible purchase or limited cash-out refinance transactions, subject to documentation and property rules. The ADU income used is limited to 30% of total qualifying income. Other programs can have different requirements. A listing’s projected rent is a starting point for questions about the unit and documentation.
Suppose your other qualifying income is $6,000 per month and the lender has already calculated $1,500 in eligible ADU rental income before applying the cap. That rent is 20% of the $7,500 combined income, so the 30% cap does not reduce it in this example. The $1,500 is an assumed underwriting figure after applicable adjustments. Ask what your program requires before counting any amount in your budget.
What to gather before making an offer
Ask how the appraiser will classify the property, whether the unit’s use and permits need verification, and which rent evidence the lender will accept. Bring any existing lease, available permit records, and the listing details. Budget separately for vacancy and upkeep if you plan to rent the unit.
See the purchase planning guide and mortgage calculator.