Short answer: on most conventional purchases of a one-unit home, the ADU’s rent counts for nothing toward your qualifying income. On an FHA loan it can count, up to 75% of the appraiser’s fair market rent, capped at 30% of your total effective income. That gap is worth real buying power, and most buyers do not find out about it until they are already in escrow.
I have had three versions of the same conversation in the last month. A buyer finds a San Diego house with a permitted ADU out back, does the math on the rent, and assumes the unit helps them qualify. In two of the three, it did not. Nothing was wrong with the property. It was the loan they had already picked.
Who this is for
San Diego buyers looking at a single-family home that already has a permitted accessory dwelling unit: a converted garage, a detached casita, a JADU carved out of the main house. After several years of ADU-friendly state law, these are common enough that you will run into one on an ordinary Saturday of showings.
This is not about building an ADU. It is about buying a house that already has one, and what that unit does and does not do to the financing.
Why the timing is unusual right now
Two numbers are moving in opposite directions. Freddie Mac’s Primary Mortgage Market Survey put the 30-year fixed national average at 6.95% on September 17, 2026, up from 6.76% the week before. A 19-basis-point move in a single week is the largest I have tracked in this survey in some time, and it is a national average for comparison only, not an offer or a quote.
At the same time, San Diego inventory has been running near 6,400 active listings and roughly 3.2 months of supply, the loosest this market has been since 2019. So payments got more expensive in the same month that buyers got more room to negotiate. That combination is exactly why ADU properties are getting a second look: buyers are hunting for something that offsets the payment.
Which makes it worth knowing, before you write an offer, whether the rent actually does that on paper.
What each loan type actually does with ADU rent
The rules are set by the agency behind the loan, not by the lender and not by the appraiser.
| Scenario | Does ADU rent count toward qualifying income? |
|---|---|
| Conventional, one-unit primary residence with an ADU | Generally no. The property is underwritten as a one-unit home. |
| FHA purchase, existing ADU with no rental history | Yes. Up to 75% of the lesser of appraiser’s fair market rent or the lease. |
| FHA 203(k), adding a new ADU | Yes, but only 50% of the lesser of market rent or lease. |
| FHA, any ADU scenario | ADU income cannot exceed 30% of total monthly effective income. |
| FHA cash-out refinance on a one-unit with an ADU | No. ADU rent is not usable as effective income. |
| True 2-4 unit property (not an ADU) | Different rules entirely. This is not an ADU question. |
The FHA treatment comes from HUD Mortgagee Letter 2023-17, effective October 2023. Guidelines change, so treat this as the framework rather than the final word on any specific file.
What that looks like in dollars
Say the appraiser reports fair market rent on the ADU at $1,800 a month, and your total monthly effective income before the ADU is $9,000.
- FHA allows 75% of $1,800, which is $1,350.
- The 30% cap on total effective income would be $2,700, so the full $1,350 survives the cap.
- Your effective income goes from $9,000 to $10,350.
On a conventional one-unit loan, the same property produces $0 of qualifying income from that unit. Same house, same tenant, same rent. The difference is entirely which agency’s rulebook your loan sits under.
Note the direction of the error most buyers make. They assume the rent helps and it does not, so they write an offer they cannot support. The fix is free and takes one phone call before you write.
Two things the appraisal has to establish
Even on FHA, the income does not appear by assertion. Underwriting needs the appraisal report plus a Single Family Comparable Rent Schedule establishing market rent. And separately, the appraiser has to treat the unit as a legitimate ADU rather than unpermitted square footage.
That second point is where San Diego deals actually die. A garage conversion done without permits is not an ADU for lending purposes, no matter how nicely it is finished or how long a tenant has lived in it. If the permit history does not support it, the rent question never even comes up, because the unit does not exist as far as the file is concerned. Ask for the permit record early, not during escrow.
When buying for the ADU is the wrong move
Three cases where I would tell you to slow down.
- You need the rent to make the payment work. If the budget only survives with a tenant in place, you have bought a vacancy risk, not a discount. Tenants turn over. Run the payment without the rent and see if you still like it.
- The ADU is the only reason the house appeals to you. ADU premiums are real and already in the price. You are usually paying for that unit up front.
- You are planning a cash-out refinance later to recoup the purchase. On FHA that ADU income is off the table for cash-out, which surprises people who mapped out a two-step plan.
Frequently asked questions
Does it matter if the ADU is already rented?
It can. An existing lease gives underwriting a second data point, and the usable figure is the lesser of that lease and the appraiser’s market rent. A lease well above market does not raise the number.
Can I count ADU income on a conventional loan ever?
Agency guidelines move, and there are narrow programs and exceptions. The safe planning assumption on a standard conventional one-unit purchase is no, then confirm against current guidelines for your specific scenario before you rely on it.
Is a JADU treated the same as a detached ADU?
Not always. Junior ADUs carved out of the existing home have their own permitting path and can be treated differently in underwriting. Get the unit classified correctly before assuming anything about the income.
What if the ADU turns out to be unpermitted?
Then it is square footage with a kitchen in it, not income. That is a separate and much bigger conversation, and it belongs to your agent and the seller before it belongs to your lender.
Where to start
If an ADU property is on your list, the order of operations matters: confirm the permit history, then find out what your loan type does with the rent, then decide what to offer. Doing it in that order costs nothing. Doing it backwards costs you the earnest money conversation.
If you want the numbers run on a specific property before you write, start a pre-qualification here and we can model it both ways, with the ADU income and without it, so you can see the actual gap.
For the broader process, my complete 2026 guide to buying a house in San Diego walks through the full timeline. If you are weighing a condo instead, condo financing has its own set of hurdles. And if student loan payments are part of your debt picture, here is how those actually get counted.
Ron Berg is a mortgage professional with The Berg Group, powered by C2 Financial Corp, serving San Diego and California. He writes about the mechanics of home financing, one topic at a time. Find more at bergequitygroup.com/blog or book a call.
Ronald Berg, NMLS #974839. C2 Financial Corp, NMLS #135622. Equal Housing Opportunity. Rates referenced are cited national averages from the Freddie Mac Primary Mortgage Market Survey as of September 17, 2026, provided for comparison only. They are not an offer, a quote, or a commitment to lend, and they are not available to any particular borrower. All loan scenarios are subject to underwriting review, program guidelines, and property eligibility. Guidelines cited are current as of publication and are subject to change. This article is educational and is not tax or legal advice.
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