The short answer: California makes ADUs easier to build than almost anywhere in the country. Cities must decide on a complete application within 60 days, can't require you to live on the property, and can't charge impact fees on units of 750 square feet or less. You can pay for one with a HELOC, a renovation loan like Fannie Mae HomeStyle, or a cash-out refinance. Once it's rented, lenders can count up to 75% of the rent, capped at 30% of your qualifying income, when you buy or refinance.
Why are so many Californians building ADUs?
Because the state made it much easier. ADU permits rose from 1,336 in 2016 to 30,354 in 2024, and ADUs were more than a quarter of all homes permitted in California in 2024 (HCD ADU Handbook). In the San Gabriel Valley, I see three reasons over and over: housing aging parents nearby, giving adult children a place to start, and adding rental income to an expensive home.
ADU permits issued in California
What does California allow on a single-family lot?
From the state's ADU Handbook (HCD):
- Up to three units in total: one ADU converted from existing space, one detached ADU, and one junior ADU (JADU) of 500 square feet or less inside the house.
- A 60-day decision on a complete application when a home already exists on the lot.
- No owner-occupancy requirement for an ADU. (A JADU that shares a bathroom with the main house is different.)
- A guaranteed minimum: cities must allow at least an 800-square-foot ADU with 4-foot side and rear setbacks.
- No impact fees on 750 square feet or less. Above that, fees are proportional to the size of the main house.
Two newer laws are worth knowing. AB 1033 lets cities adopt an ordinance allowing an ADU to be sold separately as a condominium; it isn't automatic, so check your city. SB 9 allows up to two homes, or a lot split, on most single-family lots (HCD SB 9 fact sheet).
Will an ADU raise my property taxes?
Only by the value of the new construction. The assessor adds the ADU's value to your existing assessment; your house and land keep their current Prop 13 base (Nevada County Assessor, applying statewide new-construction rules). For a long-time owner, that's a big advantage over buying a second property.
How do I pay for an ADU?
| Option | How it works | Best when |
|---|---|---|
| HELOC | Draw as the build progresses; keeps your first mortgage | You have equity and a low first-mortgage rate |
| Fannie Mae HomeStyle Renovation | Borrow against the value after the ADU is built; can finance new ADU construction (Fannie Mae) | Current equity is thin but the finished value is strong |
| Cash-out refinance | Replace your mortgage with a larger one | Your current rate is already near today's rates |
| Construction loan | Short-term loan funded in draws | Large detached builds, or when other options don't fit |
Can ADU rent help me qualify for a loan?
Yes, within limits. On a one-unit primary home with one ADU, Fannie Mae lets lenders count rent on a purchase or limited cash-out refinance. The calculation uses 75% of the rent, and ADU income can't exceed 30% of your total qualifying income. The rent has to be supported by the appraiser's rent schedule, and if you have less than a year of landlord experience, the income can only offset the housing payment (Fannie Mae B3-3.8-02). Freddie Mac's rule is similar (Freddie Mac ADU fact sheet).
That matters most when buying a home that already has a permitted ADU. An unpermitted unit generally can't be counted, and it can complicate the appraisal, so get permits sorted before you sell or refinance.
Derek\'s take
The ADU conversations I enjoy most are the multigenerational ones: a backyard unit for grandparents, or a first home for a grown child, on land the family already owns. Before you hire a designer, let's look at your equity and your first-mortgage rate together. That usually decides the financing, and the financing often decides the size of the ADU.
Frequently asked questions
Do I have to live on the property to build an ADU in California?
No. Cities can't require owner-occupancy for an ADU. A junior ADU that shares a bathroom with the main house is the exception.
How long does ADU approval take?
Cities must approve or deny a complete application within 60 days when there's already a home on the lot.
Can I sell my ADU separately?
Only if your city has adopted an ordinance under AB 1033 allowing ADUs to be sold as condominiums.
How much ADU rent can count toward qualifying?
Generally 75% of the rent, capped at 30% of your total qualifying income, on a one-unit home with one ADU.
Sources
- California HCD: ADU Handbook (March 2026)
- California HCD: SB 9 fact sheet
- Fannie Mae Selling Guide B3-3.8-02: Rental income, subject property
- Fannie Mae: Accessory dwelling units
- Freddie Mac: ADU fact sheet
- Nevada County Assessor: ADUs and property tax
This article is general education, not a commitment to lend, an offer of credit, or tax, legal or investment advice. All loans are subject to credit approval, income and asset verification, and property appraisal. Programs, rates, terms and limits change without notice; figures are current as of the publish date. Derek Shao, NMLS #242117, CA DRE #01358261. Financial Dynamics, Inc. DBA Emory Financial, Company NMLS #393986. Equal Housing Opportunity.
