Accessory dwelling units (ADUs) continue to be a hot topic across California. With ongoing updates to state housing laws through 2025, it’s easier than ever to build an ADU on your property but it’s also more complex to keep up with the rules.
This guide breaks down the 2025 California ADU law updates, what they mean for homeowners, and how these changes affect your ability to add, convert, or expand secondary units on your property.
What is an ADU?
An accessory dwelling unit (ADU) is a self-contained residential unit located on the same lot as a primary home. Common types include:
- Detached backyard cottages
- Converted garages or basements
- Attached additions with separate entrances
- Junior ADUs (JADUs) built within the primary home, typically under 500 sq ft
ADUs are sometimes called granny flats, in-law units, or casitas.
Learn more from the California Department of Housing and Community Development (HCD).
What Changed in 2025?
Several new ADU-related laws took effect on January 1, 2025:
- AB 2533 allows homeowners to legalize unpermitted ADUs or JADUs built before January 1, 2020, through a streamlined process.
- SB 1211 expands the conditions under which multiple detached ADUs can be built on the same property, especially on multi-family lots.
- AB 1332 requires cities to offer pre-approved ADU plans, reducing review times and design costs.
- AB 976 permanently bans owner-occupancy requirements. You no longer need to live on the property to rent or build an ADU.
- These updates strengthen previous laws like SB 9 and AB 2221, which already simplified setbacks, parking, and permit timelines.
You can review all current laws on California’s official ADU legislation site.
Statewide ADU Rules You Should Know
California law overrides most local ADU restrictions. The following rules apply statewide unless your local ordinance is more generous:
- Permit decisions must be made within 60 days
- Impact fees are waived for ADUs smaller than 750 square feet
- Only 4-foot side and rear setbacks are required
- No parking required if the ADU is within ½ mile of public transit or if converting a garage
- One ADU + one JADU are allowed on single-family lots
- Two detached ADUs may be allowed on some multi-family lots
- No owner-occupancy requirement for any ADU or JADU
- HOAs cannot prohibit ADUs, though they may regulate the appearance
Local governments must allow ADUs even in neighborhoods previously zoned for single-family use.
For details, see the updated HCD ADU Handbook (PDF).
Can I Split My Lot or Add Multiple Units?
Yes. Under Senate Bill 9, eligible homeowners can split a single-family parcel and build a duplex on each half — in some cases, up to four units total.
When combined with ADU laws, this means a single-family lot could legally hold multiple dwellings if zoning and site conditions allow. Local rules may still apply, but they cannot contradict state minimums.
To learn more, visit the SB 9 fact sheet.
Step-by-Step: How to Build an ADU in 2025
1. Confirm Property Eligibility
Check local zoning, setbacks, and existing structures. Most residential lots qualify, but special zones (fire hazard, floodplain, historic overlay) may require extra review.
2. Choose Your ADU Type
Decide between detached, attached, conversion, or JADU. The type affects size limits, cost, and permits.
Detached ADUs can be up to 1,200 square feet. JADUs are capped at 500 square feet and must be within the main house.
3. Use a Pre-Approved Plan (If Available)
Cities like Los Angeles, San Diego, and San Jose now offer pre-approved ADU designs you can use to speed up approvals and save on architecture fees.
This is required by AB 1332. Ask your local building department what plans are available.
4. Submit Your Permit Application
Your city has 60 days to review and approve or deny your ADU permit. Use an experienced designer or ADU contractor to ensure code compliance.
Be proactive with utility agencies — electricity, gas, and sewer connections can delay projects by months if not addressed early.
For real examples of utility-related ADU delays, see SF Chronicle’s coverage of PG&E delays.
5. Begin Construction
Once approved, construction typically takes 4 to 12 months. Work with licensed contractors and schedule inspections at each stage.
6. Final Inspection and Registration
After construction, pass final inspections and obtain a Certificate of Occupancy. Your ADU is now legal to rent or occupy.
You may also need to notify the county assessor for property tax updates.
How Much Does an ADU Cost in 2025?
Costs vary based on location, site conditions, and type of ADU.
- Garage or basement conversion: $120,000 to $200,000
- Detached ADU (700–1,200 sq ft): $200,000 to $400,000+
- Permit, utility, and design fees: Add $10,000 to $50,000, depending on the city
Detached ADUs are more expensive but allow more rental income and privacy.
Always add a 10–15% contingency to your budget for unexpected costs like soil conditions, drainage, or utility upgrades.
Financing Options for ADUs
You can finance your ADU using:
- Home equity loans or HELOCs
- Cash-out refinance
- Renovation loans (Fannie Mae’s HomeStyle or FHA 203k)
- Special ADU financing programs through state or local agencies
Some cities also offer low-interest loans or grants for affordable ADUs. Search your local housing department website for “ADU incentive programs.”
Common Mistakes to Avoid
- Not checking utility access early (especially sewer and electric)
- Relying on unpermitted contractors or skipping plan review
- Ignoring fire code in high-risk areas
- Delaying engagement with your local planning department
- Underestimating timeline — even simple builds take 9–18 months from planning to completion
You can minimize risk by hiring an ADU specialist with experience in your city.
Contact Us
Now that building and getting approved for an ADU is easier, you might consider getting one for your property. From staying close to your family to making extra income, there are a number of reasons to consider this option. If these reasons aren’t enough motivation, then maybe you’d like to turn the space into a creative studio or an at-home office. Imagine the privacy you’d have to work on your job or hobbies!
And you are in luck because, at Top Home Builders, we construct ADUs! Our home remodeling services follow all the ADU laws, and we cater to your needs so you can take hold of your future.
If you’re considering adding an ADU to your property or are thinking of converting your house into one, call us today, and we’ll take care of you.