Converting a Garage Into an ADU in Orange County: What the Work Involves

Quick answer: California law requires a city to review an ADU permit ministerially, with no hearing, and a city cannot require you to replace the parking spaces lost when a garage is converted. On the Section 66323 path, a detached garage can grow no more than 150 square feet beyond its existing dimensions, and only to accommodate ingress and egress. The build adds what a garage lacks. On our Tustin conversion that meant insulation, a dedicated electrical panel, plumbing for a kitchenette and a bathroom, and a ductless mini-split.

A garage conversion builds a dwelling inside an existing garage. Under Government Code 66323, a city in a residential or mixed-use zone must ministerially approve a building permit for one ADU and one junior ADU on a lot with a single-family home, when the unit is within the existing space of the home or of an accessory structure and the other conditions of subdivision (a)(1) are met. Both kinds of garage fit. Section 66314(d)(3) places attached garages within the primary dwelling, and a detached garage is a structure accessory and incidental to the house on the same lot, which is how Section 66313(b) defines an accessory structure.

For a detached garage, that approval may include an expansion of not more than 150 square feet beyond the existing structure's physical dimensions, and any expansion is limited to accommodating ingress and egress. The extra footage is for getting in and out of the unit, such as a new entry. The space must have exterior access, and the side and rear setbacks must be sufficient for fire and safety. A larger addition falls outside Section 66323 and is reviewed under the city's ADU standards, within the state limits in Section 66314: a detached ADU's total floor area cannot exceed 1,200 square feet, and an attached ADU's cannot exceed 50 percent of the existing home.

The size rules protect a small unit. Under Section 66321, a city cannot set a minimum ADU size that prohibits an efficiency unit, and it cannot set a maximum below 850 square feet of interior livable space, or 1,000 square feet for a unit with more than one bedroom. Section 66313(a) lists an efficiency unit as a type of ADU. Our Tustin project turned a one-car garage into a studio.

No. Under Section 66314(d)(11), when a garage, carport or covered parking structure is converted to an ADU, or demolished in conjunction with building one, the city shall not require those off-street parking spaces to be replaced.

The ADU's own parking is a separate rule. Section 66322(a) bars any parking standard for an ADU in six situations. They include an ADU that is part of the existing primary residence or an accessory structure, an ADU within half a mile walking distance of public transit, one where on-street parking permits are required but not offered to the occupant, and one with a car share vehicle within one block. A garage conversion falls in the first group whether the garage is attached or detached. Outside those situations, Section 66314(d)(10) caps the requirement at one space per ADU or per bedroom, whichever is less, and the space can be tandem parking on a driveway.

Section 66313(a) defines an ADU as a unit with complete independent living facilities, including permanent provisions for living, sleeping, eating, cooking and sanitation, and Section 66314(d)(8) holds it to the local building code requirements for detached dwellings. Our Tustin garage-to-ADU conversion shows what that takes. It was a detached one-car garage with no insulation, no dedicated electrical panel and none of the plumbing a kitchen or bathroom needs. CRS took it to the studs and rebuilt it with new insulation, a dedicated electrical panel, supply and drain lines for a kitchenette and a bathroom, and a new exterior door and window.

The finished studio has a kitchenette with quartz counters, subway tile, a gas range, an over-range microwave and a full-size refrigerator. The bathroom has a glass-enclosed shower, and the laundry is inside the unit. Luxury vinyl plank and recessed lighting run through the space, and a ductless mini-split handles heating and cooling separately from the main house's system. The job took two to three months, is fully permitted and has its own address. It was built as a rental.

A garage already has a slab, walls and a roof, which is why a conversion usually costs less than a new detached unit, though older garages sometimes need structural or foundation work first. CRS builds ADUs as part of our residential construction services. The ADU cost guide gives budget ranges for garage conversions, and the ADU permit timeline in Orange County covers how long approval takes.

State law sorts garages by where they sit. Section 66314(d)(3) groups attached garages with space located within the primary dwelling, and detached garages with structures detached from it on the same lot. A junior ADU is no more than 500 square feet of interior livable space and must be contained entirely within a single-family residence under Section 66313(d). A detached garage is not inside the residence, so it can become an ADU but not a junior ADU. Cities fill in the rest of the details within the state limits.

Tustin is a local example. Ordinance No. 1517, which the City Council adopted in December 2021, rewrote Tustin City Code 9279 to line up with state ADU and junior ADU law. According to the city's Accessory Dwelling Units page, a studio or one-bedroom ADU can reach 850 square feet, a unit with two or more bedrooms can reach 1,200, and neither an ADU nor a junior ADU can be smaller than 220 square feet.

Tustin treats a converted ADU as one built inside permitted existing interior space, either in an accessory structure or in part of the house, and it counts attached garages and storage areas as part of the house. Its junior ADU tops out at 500 square feet and has to fit completely inside the walls of the single-family house, which includes an attached garage. In Tustin, then, an attached garage can be converted to an ADU or a junior ADU, and a detached garage to an ADU only. Our page on an ADU contractor in Tustin has the Planning Division's contact details and more on our Tustin project.

Start with setbacks. Section 66314(d)(7) says no setback can be required for an existing living area or accessory structure converted to an ADU, or for a structure built in the same location and to the same dimensions as an existing one. For an ADU that is neither, the city can require no more than 4 feet from the side and rear lot lines. Under (d)(12) and Section 66323(d), an ADU cannot be required to install fire sprinklers if they are not required for the primary residence, and (d)(12) adds that building the ADU does not trigger sprinklers in the existing home.

Occupancy is where a garage differs from other space. Section 66314(d)(8) says building an ADU is not a Group R occupancy change under the local building code unless the building official makes a written finding, based on substantial evidence in the record, that it could have a specific, adverse impact on public health and safety. The same paragraph lets a city change the occupancy code of space that was uninhabitable or only permitted for nonresidential use and was later converted to residential use. A garage permitted as a garage fits that description, so the city can reclassify the converted space. If a detached garage is being demolished and replaced by an ADU, the demolition permit is reviewed with the ADU application and issued at the same time, and no written notice or posted placard is required unless the property is in an architecturally and historically significant historic district (Section 66314(e)-(f)).

Some existing problems on the lot cannot be used to deny the permit. Under Section 66322(b), a city cannot deny an ADU permit over nonconforming zoning conditions, building code violations or unpermitted structures that do not present a threat to public health and safety and are not affected by the ADU construction. That protection stops at conditions the construction does not affect, so unpermitted work inside the garage being converted is outside it. Section 66323(c) adds that a city cannot require the correction of nonconforming zoning conditions, meaning physical improvements that do not meet current zoning standards, as a condition of ministerial approval.

Under Section 66317(a), an ADU application is considered and approved ministerially, without discretionary review or a hearing. The city has 15 business days to tell you in writing whether the application is complete, and an incomplete notice must list the missing items and how to fix them. With an existing home on the lot, the city must approve or deny within 60 days of receiving a complete application, or the application is deemed approved. A denial must come back in writing with a full set of comments on what is deficient and how to remedy it.

Coastal lots add one layer. The Coastal Act still applies in the Coastal Zone, but under Section 66329 the city must approve or deny the coastal development permit for an ADU within 60 days of a complete application, is not required to hold a public hearing, and runs that review concurrently with the ADU permit. If the city has no certified local coastal plan or program, the Coastal Commission approves or denies the coastal permit within 60 days of a complete application.

Yes, but you cannot sell it separately. Section 66314(d)(1) says an ADU may be rented separately from the primary residence but shall not be sold or otherwise conveyed separately, except as provided in Article 4, which starts at Section 66340. Rental length depends on how the unit was approved. For an ADU approved under Section 66323, subdivision (e) requires any rental to be for a term longer than 30 days. For other ADUs, Section 66315 lets the city require rentals of 30 days or longer.

On a lot with a single-family home, Section 66315 bars an owner-occupant requirement for an ADU, so the city cannot make living on the property a condition of the permit. A junior ADU in an attached garage must also meet Article 3, which starts at Section 66333, and our post on ADU rental income in Orange County covers the owner-occupancy rule that applies to some junior ADUs, along with what a rental unit earns.

This is general information, not professional advice.

CRS Construction is a licensed Orange County general contractor (CSLB #1021647) based in Fountain Valley, and we built the permitted Tustin studio described above. To start yours, request a quote with the garage's size, whether it is attached or detached, and what you want it to become. We begin with a visit to the property and a written scope.