California ADU rules let you build an accessory dwelling unit on almost any residential lot in the state, and they cap how restrictive your city or county is allowed to be. Local governments can write their own ordinances, but those ordinances cannot go beyond the statewide limits on size, height, setbacks, fees, parking, or review time. Where a jurisdiction has no ADU ordinance, the state standards apply on their own.
Size, Height, and Setback Limits Your City Can’t Exceed
A detached ADU can be up to 1,200 square feet. An attached ADU can be up to 50 percent of the primary home’s floor area, with a floor of 800 square feet guaranteed regardless of that percentage. No local agency may set a maximum below 850 square feet for a studio or one-bedroom, or below 1,000 square feet for a unit with two or more bedrooms.1California Department of Housing and Community Development. Accessory Dwelling Unit Handbook At the small end, local minimums cannot prohibit efficiency units, so ADUs as small as 150 square feet are possible depending on jurisdiction.
Height limits for detached ADUs run on a tiered system. The baseline is 16 feet. On lots within a half-mile walking distance of a major transit stop or high-quality transit corridor, the limit rises to 18 feet, with two additional feet allowed to match the primary dwelling’s roof pitch. The same 18-foot limit applies on lots with an existing or proposed multifamily, multistory building. Attached ADUs can reach 25 feet or the height of the primary dwelling, whichever is lower, though local agencies are not required to allow more than two stories.1California Department of Housing and Community Development. Accessory Dwelling Unit Handbook
Setbacks are generous. If you are converting an existing structure, no setback is required at all. For new construction outside that footprint, the maximum a local agency may require is four feet from the side and rear lot lines.2California Legislative Information. California Government Code 65852.2 Local agencies also cannot use development standards like floor area ratio, open space, or minimum lot size to block an 800-square-foot ADU that otherwise meets the four-foot setback.
Impact Fees and Utility Connection Charges
The fee rules save real money. ADUs under 750 square feet of interior living space are fully exempt from impact fees. ADUs of 750 square feet or more must be charged impact fees proportional to the ADU’s square footage relative to the primary dwelling, so the number comes in well below what a standalone home would trigger.2California Legislative Information. California Government Code 65852.2
Utility hookup rules depend on the build type. For a conversion within an existing structure, such as a garage or basement, the local agency cannot require a new or separate utility connection or charge a connection fee. For a detached, newly built ADU, a connection may be required, but any fee must be proportionate to the unit’s size or the burden it places on the system.2California Legislative Information. California Government Code 65852.2
Construction is the real line item. Garage conversions in California typically run $90,000 to $140,000, attached new construction $120,000 to $190,000, and detached new builds $160,000 to $300,000 or more depending on size and finishes. A current site survey, needed for accurate plans, generally costs several hundred to over a thousand dollars. Every ADU must include a kitchen with a cooking appliance and permanent sanitation facilities to qualify as an independent living space.
How the Permit Process Works
ADU permits in California are approved ministerially. Your local agency reviews the application against objective standards without a public hearing or discretionary judgment call. If it meets the rules, it gets approved.
The agency has 60 days from receiving a complete application to approve or deny it. Miss that window and the application is deemed approved by operation of law.2California Legislative Information. California Government Code 65852.2 An incomplete application restarts the clock, so documentation matters.
Before you draft plans, confirm your property lines with a professional survey, locate utility lines, and verify emergency vehicle access. Plans must comply with state and local standards for size, height, and setbacks, and a Title 24 energy compliance report is required as part of the application. If you are seeking local fee waivers or affordability incentives, your jurisdiction may require you to record a deed restriction limiting occupancy or rental terms.
After the permit issues, construction requires inspections at the foundation, framing, electrical and plumbing rough-in, insulation, and final stages. The project closes with a final inspection, after which the local jurisdiction issues a Certificate of Occupancy — the legal clearance to inhabit or rent the unit.
Who Can Live There and Whether You Can Rent It
California does not require owner-occupancy for ADUs. You can rent out both the primary house and the ADU without living on the property yourself.1California Department of Housing and Community Development. Accessory Dwelling Unit Handbook This is a permanent rule; the earlier sunset date on the prohibition was removed by the legislature.
Short-term rentals are treated differently. Local agencies can require that ADUs be rented for terms longer than 30 days, and ADUs approved through certain state provisions must be rented for longer than 30 days regardless of local rules. Check your local ordinance before listing anywhere.
Selling the ADU separately from the main house is possible only where your city has opted in. AB 1033, effective October 2023, authorized local governments to allow ADUs to be sold independently as condominiums, but your city or county must adopt a local ordinance specifically allowing it. Where no such ordinance exists, the ADU stays tied to the primary property.
Junior ADUs as a Smaller Alternative
A junior accessory dwelling unit is a different animal and worth knowing about if you are weighing options. JADUs max out at 500 square feet and must be built entirely within the walls of an existing single-family home, including an attached garage.3California Legislative Information. California Government Code 65852.22 A JADU needs cooking facilities with a sink but can share a bathroom with the main house.
JADUs are exempt from impact fees and cannot trigger utility connection fees.4California Department of Housing and Community Development. Frequently Asked Questions – Junior Accessory Dwelling Units No parking can be required. In exchange, a JADU cannot be sold separately from the primary home, cannot be used as a short-term rental, and must be rented for more than 30 days if rented at all. As of January 1, 2026, owner-occupancy for a JADU depends on the bathroom setup: if the JADU shares sanitation with the main house, the owner must live in either the main home or the JADU; if the JADU has its own bathroom, no owner-occupancy is required.1California Department of Housing and Community Development. Accessory Dwelling Unit Handbook You can have both a JADU and a full ADU on the same single-family lot.
Grants and Financing
The California Housing Finance Agency’s ADU grant program reimburses eligible homeowners for up to $40,000 in predevelopment costs: architectural plans, site preparation, soil tests, Title 24 reports, permit fees, and other non-construction expenses.5California Department of Housing and Community Development. Funding for ADUs The money can also go toward an interest rate buy-down on a construction loan. It does not cover labor or building materials. Eligibility requires income below 80 percent of the Area Median Income for the county and owner-occupancy of the property as a primary residence.6CalHFA. ADU Grant Program
The catch: as of December 2023, CalHFA announced that the latest round of ADU grant funding had been fully allocated.6CalHFA. ADU Grant Program The program’s website does not indicate when or whether new funding will become available. If you are counting on this grant, contact a CalHFA-approved participant to ask about current availability before investing in plans. Some local governments offer their own ADU incentives, including low-interest loans, local fee waivers, or technical assistance, so check with your city or county housing department as well.
For construction financing, standard loan products work. Fannie Mae treats an ADU like any other home improvement; borrowers who qualify for a HomeReady loan can include ADU rental income to help with qualification.7Fannie Mae. Accessory Dwelling Units An FHA 203(k) renovation loan can wrap ADU construction into the purchase of a fixer-upper. Home equity lines of credit and cash-out refinances are common routes for homeowners with enough equity in their existing property.
Insurance and Property Tax Effects
Building an ADU changes your insurance picture, and most policies require you to notify the carrier before construction. An attached ADU or interior conversion is typically covered as part of the main dwelling under an existing homeowners policy. A detached ADU usually falls under “other structures” coverage, which is often limited to around 10 percent of your dwelling coverage, rarely enough to rebuild a standalone unit after a loss.
If you plan to rent the ADU, you may need landlord or rental property insurance for tenant-related risks and liability. Short-term rental hosting typically requires either a home-sharing endorsement or a separate landlord policy. Umbrella coverage adds excess liability protection above base policy limits. Call your insurer before construction starts and describe the project in detail: attached or detached, who will live there, and whether you will rent it out.
For property taxes, Proposition 13 protects your existing assessment. The county assessor reassesses only the value of the new construction, not your entire property, so your existing home’s assessed value stays locked at its Prop 13 base. The ADU gets assessed at its construction value and added to your tax bill. For a modest unit, the annual property tax increase is often small compared to the rental income the unit generates.