The mechanism your broker described is Fannie Mae's policy, effective March 2026, that lets a buyer count projected ADU rental income toward qualifying income on an owner-occupied one-unit purchase, capped at 30 percent of qualifying income. Confirm the current terms and the cap in writing with your lender, since guidelines get updated and individual lenders layer their own conditions on top.
Where the projected number comes from: an appraiser estimates market rent for the unit, usually on a form built for that purpose, based on what comparable units in the area rent for. It isn't your guess and it isn't the seller's number. Lenders typically apply a vacancy factor, so a $1,600 market rent estimate doesn't add $1,600 to your income, it adds something less.
The cap works as a ceiling on how much of your total qualifying income can come from that rent. So if the unit's counted rent is large relative to your salary, you hit the limit and the extra doesn't help. That means the rule does the most for buyers whose own income is decent but not quite enough, and less for buyers with very little other income.
On your second question, it's aimed at a unit that exists and can be rented, not a unit you plan to build later. Renovation-type loans are a different product with different rules, and that's a separate conversation with the lender.
One piece people miss: qualifying with projected rent means your budget assumes a tenant from month one. If it takes three months to lease, you carry the full payment alone, so hold reserves that cover that gap before you count on the income.
