Yes, you can buy an Orange County home that already has an accessory dwelling unit (ADU), and on most residential lots with a single-family home you can add one later. California law requires cities to approve certain ADUs and limits how far local rules can restrict them.
For a home with an existing unit, the key question is whether it was permitted and passed final inspection, because that affects financing, appraisal, insurance and how you can use it. If you plan to build, check feasibility during your contingency period.
The state ADU rules in brief
California's ADU law sits in Government Code sections 66310 through 66342, where a 2024 bill, SB 477, moved it from older sections such as 65852.2. The state Department of Housing and Community Development (HCD) enforces it. Cities can adopt their own ADU ordinances but cannot go below the state minimums.
On a lot with a single-family home, a city must allow, in combination, one ADU converted from existing space (part of the house or an existing structure such as a garage), one junior ADU (JADU), and one new detached ADU of up to 800 square feet with side and rear setbacks of no more than four feet. Other key rules:
- Size. A JADU is limited to 500 square feet of interior livable space inside the house, which can include an attached garage. A city cannot cap ADU size below 850 square feet, or 1,000 square feet for more than one bedroom.
- Setbacks and height. Side and rear setbacks cannot exceed four feet, and converted space needs none. Detached ADUs must be allowed at least 16 feet of height, more near major transit. Attached ADUs can reach 25 feet or the main home's height limit, whichever is lower.
- Parking. No more than one space per ADU or per bedroom, whichever is less, and none in several cases, such as within a half-mile walk of public transit. Converting a garage does not require replacing its parking.
- Owner-occupancy. Cities cannot require the owner to live on a property with an ADU. Under a law effective January 1, 2026, a JADU requires owner-occupancy only when it shares sanitation facilities, such as a bathroom, with the house.
Buying a home that already has an ADU
Confirm it was permitted and finaled
A converted garage with a kitchenette is not the same as a permitted ADU. State law requires a certificate of occupancy before anyone lives in an ADU or JADU, so ask the seller for the building permit, final inspection sign-off and certificate of occupancy. Then pull the permit history from the city to confirm the permits cover a dwelling unit, not just a garage or recreation room.
A JADU also comes with a recorded deed restriction that bars selling it separately from the house and binds future owners, so ask your title officer for it. Have your home inspector examine the unit as closely as the main house.
If the unit was never permitted
State law gives owners a path to legalize older units. Under Government Code section 66311.7, a city cannot deny a permit for an ADU or JADU built before January 1, 2020, just because it violates building standards or local ADU rules, unless a correction is needed to fix conditions that make a building substandard under the Health and Safety Code. Cities must publish a checklist of those conditions and tell owners they can get a confidential code inspection from a licensed contractor before applying. With limited exceptions, owners do not pay impact fees or utility connection charges to legalize such a unit, and the city cannot penalize them for having had it.
Separately, Health and Safety Code section 17980.12 lets the owner of a qualifying unit request a five-year delay in enforcement of building standards when the correction is not necessary to protect health and safety. Agencies can approve those requests until January 1, 2030.
None of this makes an unpermitted unit free to fix. Before you remove contingencies, get a contractor's inspection, ask the city what legalization would involve, and price the purchase accordingly.
How appraisers and lenders treat the unit
Under Fannie Mae's appraisal rules, the appraiser reports an ADU's living area separately from the main home's square footage and analyzes the unit's effect on value and marketability. For an addition built without a required permit, the appraiser must comment on the quality of the work and any effect on value. Fannie Mae's guidelines allow only one ADU on a one-unit property, so if a lot has more than one extra unit, raise it with your lender early.
Since October 2025, Fannie Mae has allowed rental income from an ADU on a one-unit principal residence for purchases and limited cash-out refinances. Only one ADU's rent counts, capped at 30% of your total qualifying income, and the usual rental documentation applies. HCD notes that FHA and Freddie Mac also let ADU rent count under their own rules. Our financing team can help.
Tenants, taxes and insurance
If the ADU is occupied, get the lease, rent history and deposit records early. A lease can transfer to you at closing, and California tenant protections may limit when a tenancy can end, so get legal advice before planning to use the unit yourself.
Buying the home is a change in ownership, so the assessor reassesses the whole property, ADU included, at market value. Our guide to Orange County property taxes explains how the new bill is built.
The California Department of Insurance explains that "other structures" coverage protects structures not attached to the house and is normally limited to 10% of the dwelling limit, though you can buy more. That may not be enough for a detached ADU, so ask your insurer how the unit is covered and tell them if you plan to rent it.
Buying with a plan to add an ADU
Feasibility checks before you buy
Do these during escrow, while contingencies are in place:
- Local rules. Read the city's ADU page and ordinance. In Santa Ana, for example, the city posts pre-approved ADU plans and its development standards online.
- Site layout. Look for room for an 800-square-foot unit with four-foot side and rear setbacks, or a garage you could convert. Check the preliminary title report for easements.
- Utilities. Ask the water and sewer providers about connection requirements, and an electrician whether the service can support another unit.
- Slope and soils. Hillside lots may need geotechnical work or grading.
- Coastal zone. Near the coast, an ADU may also need a coastal development permit, which must be decided within 60 days of a complete application when a home already exists on the lot.
- HOA. Under Civil Code sections 714.3 and 4751, CC&Rs cannot prohibit or unreasonably restrict an ADU or JADU on a lot zoned for single-family use. An HOA can apply reasonable objective standards, such as materials or architectural style, but not fees or other financial requirements.
Fees
An ADU with 750 square feet or less of interior livable space is exempt from impact fees. Above that, fees must be proportional to the ADU's size compared with the main house. School districts cannot charge fees on ADUs or JADUs under 500 square feet. A converted ADU or JADU generally cannot be required to have its own new utility connection, and connection charges for other ADUs must be proportionate to the unit's size or plumbing fixtures.
Rough timeline
- Research and design. Confirm feasibility, then hire a designer or choose a pre-approved plan. Every city and county must have a program to pre-approve ADU plans.
- Application. The city has 15 business days to say whether your application is complete. Once it is, the city must approve or deny it within 60 days when a home already exists on the lot, or within 30 days for a detached ADU using a pre-approved plan. If the city misses the 60-day deadline, the application is deemed approved.
- Construction and inspections. Build to the permitted plans and pass the city's inspections.
- Certificate of occupancy. No one can live in the unit until the city issues it.
- Reassessment. The assessor values the completed construction, and the increase typically arrives as a supplemental tax bill.
Under the State Board of Equalization's Property Tax Rule 463, only the newly constructed portion gets a new base year value. Your existing home and land keep their assessed value. A garage conversion can also count as new construction because it changes how that space is used.
Renting, selling separately and SB 9
ADUs can be rented. JADUs, and ADUs approved under the state's streamlined categories, must be rented for terms longer than 30 days, and cities can require rentals of 30 days or longer for other ADUs. Check your city's short-term rental rules too. Rental income is never guaranteed, so plan for vacancies, maintenance and turnover.
Normally an ADU cannot be sold separately from the main house. Under Government Code section 66342, a city may choose to allow the home and ADU to be sold separately as condominiums, with conditions that include a safety inspection, lienholder consent and, in an existing HOA, the association's written authorization. It is optional, so ask your city.
SB 9 is a separate state law that can allow a lot split or additional primary units on eligible single-family lots. HCD explains that after an SB 9 lot split, each resulting lot is limited to two units, ADUs and JADUs included.
This is general information, not legal or tax advice. Confirm the details for a specific property with your city, a California real estate attorney, a CPA and your insurer.
How Remark Homes can help
We help Orange County buyers check permit history before they commit, build ADU feasibility questions into their contingency period, and find lenders who understand ADU income. Learn more about buying with Remark Homes, or get started when you are ready to tour homes.
Frequently asked questions
Can I build an ADU on my Orange County lot?
On most residential lots with a single-family home, yes. State law requires cities to approve a converted ADU, a JADU and a detached ADU of up to 800 square feet, though site conditions, utilities and the coastal zone shape what fits.
Do I have to live on the property if it has an ADU?
Not for an ADU, because cities cannot impose an owner-occupancy requirement. A JADU that shares sanitation facilities with the main house does require the owner to live in the house or the JADU.
Will a lender count rent from the ADU?
Often, within limits. Fannie Mae allows rent from one ADU on a one-unit principal residence for purchases and limited cash-out refinances, capped at 30% of total qualifying income.
Does adding an ADU reassess my whole property?
No. Only the new construction receives a new base year value, and your existing home and land keep their assessed value.
Sources
- California Department of Housing and Community Development: Accessory Dwelling Unit Handbook
- Fannie Mae: Selling Guide Announcement SEL-2025-08
- Fannie Mae: Selling Guide B4-1.3-05, Improvements Section of the Appraisal Report
- California State Board of Equalization: Property Tax Rule 463, Newly Constructed Property
- California Department of Insurance: Residential Property Insurance Guide
- City of Santa Ana: Accessory Dwelling Units
