The short answer: on a $1.1 million Orange County home with an existing ADU, the ADU’s rent covers roughly 25% to 53% of your total monthly payment at today’s rates. Where you land in that range depends on three things: how much you put down, how big the ADU is, and which unit you live in.
That’s less than the “the tenant pays your mortgage” pitch you’ll see on social media. It’s also a lot more than nothing. On a typical deal, it’s $2,000 to $3,500 a month that you’re not paying out of your own paycheck, plus the ADU rent can help you qualify for the loan in the first place.
Below is the actual math. Every assumption is on the table so you can swap in your own numbers.
The assumptions
| Item | Assumption |
|---|---|
| Purchase price | $1,100,000, single-family home with a permitted ADU |
| Loan | 30-year fixed, owner-occupied |
| Rate | 7.40% |
| Property tax | 1.15% of price (about $1,054/month) |
| Homeowners insurance | $250/month |
| PMI (under 20% down) | About 0.5% of the loan per year |
| Vacancy + maintenance reserve | 10% of rent |
The 7.40% rate is Freddie Mac’s Primary Mortgage Market Survey average for the week of October 8, 2026. Your quote will differ based on credit and points, but that’s the realistic starting line right now. I wrote about what 7%+ rates mean for ADU properties this fall, including why buyers have more negotiating room than they did a year ago.
Property tax in California starts at a 1% base under Prop 13, plus local voter-approved bonds and assessments. Most Orange County cities land somewhere between about 1.05% and 1.25% all in. Check the specific parcel’s tax bill, especially for Mello-Roos in newer tracts.
On loan size: $1.1 million with 5% or 10% down still fits under the 2026 Orange County conforming limit of $1,249,125, per FHFA’s conforming loan limit values. That matters because it keeps you out of jumbo pricing and jumbo down payment rules.
Your monthly payment at each down payment
| Down payment | Loan amount | Principal + interest | Tax | Insurance | PMI | Total monthly |
|---|---|---|---|---|---|---|
| 5% ($55,000) | $1,045,000 | $7,235 | $1,054 | $250 | $435 | $8,975 |
| 10% ($110,000) | $990,000 | $6,855 | $1,054 | $250 | $412 | $8,571 |
| 20% ($220,000) | $880,000 | $6,093 | $1,054 | $250 | $0 | $7,397 |
Under 20% down, private mortgage insurance adds about $400 a month on this loan. It protects the lender, not you, and it comes off once you build enough equity.
How much the ADU covers, by ADU size
ADU rents below come from my Orange County ADU rent guide by square footage, using mid-range numbers for standard finishes:
- 1 bed, about 550 sq ft: $2,300
- 2 bed, about 800 sq ft: $2,800
- 3 bed, about 1,100 sq ft: $3,300
Gross coverage (rent ÷ total payment)
| ADU | 5% down | 10% down | 20% down |
|---|---|---|---|
| 1 bed, $2,300 | 25.6% | 26.8% | 31.1% |
| 2 bed, $2,800 | 31.2% | 32.7% | 37.9% |
| 3 bed, $3,300 | 36.8% | 38.5% | 44.6% |
What you actually pay out of pocket each month
This version holds back 10% of the rent for vacancy and repairs, which is the number I’d budget around.
| ADU | 5% down | 10% down | 20% down |
|---|---|---|---|
| 1 bed, $2,300 | $6,905 | $6,501 | $5,327 |
| 2 bed, $2,800 | $6,455 | $6,051 | $4,877 |
| 3 bed, $3,300 | $6,005 | $5,601 | $4,427 |
So on the most common setup I see, a 2-bed ADU with 10% down, your effective housing cost drops from about $8,571 to about $6,051. That’s roughly $30,000 a year back in your pocket.
Two things stand out:
- ADU size moves the needle more than you’d think. Going from a 1-bed to a 3-bed ADU adds about 12 points of coverage. That’s more than going from 5% down to 20% down on the 1-bed. When you’re comparing listings, the bedroom count on the ADU is worth real money.
- Down payment moves it too, but it costs you. Getting from 10% to 20% down saves about $1,170 a month (lower loan plus no PMI), but it takes another $110,000 in cash.
The bigger move: live in the ADU, rent the main house
Most buyers assume they’ll live in the main house. Run it the other way and the numbers change a lot.
A 3 to 4 bedroom main house on a standard Orange County lot rents for around $3,900 a month (the same figure I used in what it costs to buy a home and add an ADU in Orange County).
| Scenario (10% down, $8,571 payment) | Rent coming in | Coverage | Your out of pocket (after 10% reserve) |
|---|---|---|---|
| You live in the house, rent the 2-bed ADU | $2,800 | 32.7% | $6,051 |
| You live in the ADU, rent the main house | $3,900 | 45.5% | $5,061 |
| After year 1: move out, rent both units | $6,700 | 78.2% | $2,541 shortfall |
Living in the ADU gets you close to half the payment covered. It’s a smaller space, but for a lot of my buyers it’s a one or two year sacrifice that saves nearly $12,000 a year compared to living in the house.
And the third row is where house hacking actually builds wealth. Most owner-occupied loans require you to live there about a year. After that, many of my buyers rent both units, buy the next owner-occupied property with a low down payment, and repeat. I lay out that whole sequence in How to Start House Hacking With an Existing ADU.
Notice that even with both units rented, a $1.1M property at 10% down and 7.40% doesn’t fully cash flow. That’s the honest picture at today’s rates. It’s a property where tenants pay most of the cost while you build equity on a $1.1M asset, and your numbers improve every time you raise rents or refinance into a lower rate.
What gets you to 50% or more while living there
If you want the ADU alone to cover half your payment while you live in the main house, here’s what has to be true on a $1.1M purchase:
- At 10% down, you need about $4,290 a month in ADU rent. That’s a large 3-bed or a property with two ADUs.
- At 20% down, you need about $3,700. A well-finished 3-bed ADU in a strong rental city can get there.
Or you lower the purchase price. In my September Santa Ana report, the median active ADU listing was $1,050,000, and several were under $1 million. Garden Grove ADUs rent for $2,350 to $3,800 depending on size, per my Garden Grove ADU rent breakdown. A $950,000 home with a $2,800 ADU puts you about 5 points higher on coverage than the same ADU on a $1.1M home.
The ADU also helps you qualify, not just pay
Coverage is only half the benefit. The other half happens before you close.
Fannie Mae’s Desktop Underwriter 12.1 update lets ADU rent count as qualifying income on a one-unit primary residence, capped at 30% of your total qualifying income. FHA does something similar under HUD Mortgagee Letter 2023-17, counting 75% of the ADU’s rent.
On a $2,800 ADU, that’s about $2,100 a month of extra qualifying income. For a household earning $25,000 a month, that can add roughly $100,000 or more of purchasing power at today’s rates. I ran the full version in How Much More House Does an ADU Let You Qualify For?
One catch: the ADU has to be permitted. Lenders won’t count rent from a unit that isn’t legal, and the coverage math above only holds if you can actually rent it out with a clean lease.
Costs that eat into your coverage
Don’t skip these when you build your own version:
- Utilities. If the ADU shares meters with the house, you’re either absorbing part of the tenant’s usage or billing it back. Separate meters are worth paying attention to on a listing.
- Insurance. Your policy needs to cover a rental unit on the property. California homeowners premiums have been climbing, so get a real quote before you write an offer.
- Rent growth limits. Many ADUs built in the last 15 years are exempt from California’s statewide rent cap, but not all, and some cities have their own rules. Read whether a new ADU is exempt from AB 1482 before you assume rents will rise freely.
- Repairs and turnover. The 10% reserve I used is a floor, not a ceiling. Here’s what to budget for on your first investment property.
There’s also an upside most buyers forget. When you rent part of a home you live in, the IRS lets you split expenses between personal and rental use and depreciate the rental portion. IRS Publication 527 covers the rules. On a $1.1M property, depreciation on the ADU’s share can offset a good chunk of the rental income on paper. Talk to your CPA early.
Frequently Asked Questions
How much of my mortgage can an ADU cover in Orange County?
At 7.40% rates on a $1.1M home, ADU rent typically covers about 25% to 45% of the full payment (principal, interest, tax, insurance and PMI) while you live in the main house. If you live in the ADU and rent the main house, it’s closer to 43% to 53%.
Can an ADU pay my entire mortgage?
Not on a typical Orange County purchase at today’s rates while you live there. With both units rented after you move out, coverage on a 10% down $1.1M purchase is around 78%. Full coverage usually takes a larger down payment, a lower purchase price, or a property with more than one rental unit.
Is it better to live in the main house or the ADU?
Financially, living in the ADU and renting the main house almost always covers more of the payment, because the house rents for more. It comes down to how much space you need and for how long.
Does ADU rent count toward qualifying for the loan?
Yes, on many loans. Fannie Mae and FHA both allow it on a one-unit primary residence if the ADU is permitted, generally counting 75% of market rent, with Fannie Mae capping it at 30% of total qualifying income.
Run Your Own Numbers With Me
Every one of these numbers moves with the specific property: the ADU’s size and finishes, the city’s rents, the tax rate and your loan. If you want me to run the coverage math on the homes in your price range, I’ll pull the real rents and build it out with you.
Call Dylan Serna, ADU Realtor at 714-860-2868.
Dylan Serna | The ADU Realtor | eXp Realty | DRE #02217359
This post is general information, not financial, lending or tax advice. Rates, rents, taxes and insurance costs are estimates as of October 2026 and will differ on any real property. Talk to a licensed lender and your CPA before you buy.
