Can ADU rental income help me qualify for a mortgage?

Yes, but Fannie Mae sets tight limits

An accessory dwelling unit (ADU) out back can feel like a little money machine. The question is whether your lender agrees.

Under Fannie Mae's Selling Guide, rental income from an ADU can count toward qualifying, with these limits:

• One ADU only. Rental income is allowed from only one existing ADU.
• Certain loans only. It applies only to a purchase or a limited cash-out refinance. A cash-out refinance does not get this treatment.
• One-unit primary residence. The ADU must go with a one-unit home you will live in as your primary residence.
• 30% cap. The qualifying rental income from the ADU is limited to 30% of your total qualifying income.

Notice the word existing. A cottage you plan to build next summer is a great idea and a nice drawing. It is not qualifying income.

These are Fannie Mae rules. FHA, VA, Freddie Mac, jumbo and non-QM lenders set their own, and each lender can add overlays. Ask your loan officer which program you are on before you count the rent.

How the 30% cap works, with made-up numbers

The cap is measured against total qualifying income, and the ADU rent is part of that total.

Say a buyer earns $7,000 a month from a job, and the lender counts $3,000 a month from the ADU. Total qualifying income is $10,000. Thirty percent of $10,000 is $3,000, so the ADU income fits right at the cap.

Now say the job pays much less and the ADU rent stays the same. The ADU would be more than 30% of the total, so the lender limits how much of it counts. The cottage is pulling its weight, but Fannie will not let it carry the whole couch up the stairs.

These numbers are hypothetical. Your lender figures the actual qualifying amount from your documents, and the calculation can involve more steps than this example shows. The point is that ADU rent helps, but it cannot be most of your income on a Fannie loan.

What counts as an ADU

Fannie Mae defines an ADU as an additional living area, typically independent of the main home, on the same lot as a one-unit dwelling. Only one ADU is allowed on that lot, and ADUs are not allowed with a two- to four-unit property.

To count as an ADU, the space needs:

• Its own way in and out. It can share access with the main house, but it is not an ADU if the only way in is through the main house.
• A kitchen with cabinets, a countertop, a sink with running water and a stove or stove hookup. A hot plate, microwave or toaster oven does not count.
• Its own sleeping area, bathing area and bathroom.

Fannie says the appraiser determines whether the unit meets the definition. An ADU allowed under current zoning is considered legal. Some older units are legal nonconforming because they were built before the zoning changed. If an ADU is not allowed under zoning, the loan can still be eligible in some cases, if the lender confirms it will not jeopardize a future insurance claim.

Where the appraisal comes in

The rent has to be supported. For a purchase, Fannie requires the appraiser's rent schedule (Form 1007, or Form 1025 when applicable) and a fully signed lease if one is being transferred to you. For a refinance, the lender uses the rent schedule plus your most recent tax return with Schedules 1 and E, and the current lease only in certain situations.

If market rents do not reasonably support the rent you report, Fannie says the lender must document more about the income, provide a written analysis, or use the lower amount.

Here is what I see in the field. The ADU and the main house are separate things. A detached ADU never counts in the main house's square footage. I measure and value it on its own, using what similar properties with ADUs actually sold for and what similar units actually rent for. A sales flyer that says "mortgage helper!" is enthusiasm, not evidence.

Good rental comps for ADUs can be thin, so the more you can document, the better.

Keep two questions apart. Whether the rent helps you qualify is a lending rule. How much value the ADU adds to the property is a market question, and it is measured from local sales, not from a national rule of thumb. An ADU can help on one and do little on the other, depending on the loan program and the neighborhood. A cottage that rents well in a college town may add less value on a street where buyers just want a bigger yard.

If you are buying, ask the listing agent early for the lease, the rent history and any permits. Those papers do a lot more for your loan file than the word "income" in the listing remarks.

What to do before you count on the rent

1. Ask your loan officer which program you are on and whether it allows ADU rent.
2. Confirm the unit qualifies: separate entrance, full kitchen with a stove or hookup, and its own bathroom.
3. Check permits and zoning with the city or county, and keep copies.
4. Collect the lease and any rent payment records. For a purchase, get the seller's lease if the tenant is staying.
5. Gather rent evidence: listings and leases for similar units nearby. Share it with the appraiser.
6. Run the 30% math with your loan officer before you write an offer, so the cottage does not surprise you in underwriting.