The garage is the most common place Monterey Peninsula homeowners look when they want an ADU. Some want to convert it. Many want to keep it and build on top of it: the garage stays a garage, and a one-bedroom unit sits above it with its own stair and its own front door.
It is a good idea on a small lot, because it adds a dwelling without giving up yard or parking. It is also the version of an ADU that the state’s height rules handle least generously, and the one where the existing structure most often turns out not to be what the drawings need it to be.
Here is how an ADU over a garage works under California law, why “attached” or “detached” decides almost everything, and what has to be true of the garage underneath before anyone draws a floor plan.
Can You Build an ADU Over a Garage in California?
Yes. State law treats an ADU as a unit that can be detached, attached to the primary home, or created inside existing space, and nothing in it rules out a unit built above a garage. The question is never whether the idea is allowed. It is how tall the result is allowed to be, and that depends on whether the garage is attached to your house.
A second story needs height. A garage with a habitable floor above it typically needs a total building height well past 16 feet once you account for the garage ceiling, the floor structure, an eight-foot living space and a roof. That number is the one to keep in mind for everything that follows.
Attached or Detached: The Rule That Decides the Project
California sets minimum height allowances that every local ADU ordinance has to honor. Cities and counties can allow more; they cannot allow less.
| ADU type | Height the state guarantees |
|---|---|
| Detached ADU, single-family lot | 16 feet |
| Detached ADU within half a mile of major transit | 18 feet, plus up to 2 more feet to match the roof pitch of the main house |
| Detached ADU on a lot with a multistory multifamily building | 18 feet |
| Attached ADU | 25 feet, or the height limit for the primary dwelling, whichever is lower |
Source: Government Code section 66321(b)(4), as summarized in the state’s ADU Handbook (March 2026 update).
Read that table with a two-story garage in mind and the answer is plain:
- Over an attached garage, the ADU is an attached ADU. The state guarantees up to 25 feet or your zone’s height limit. A second story over the garage usually fits inside that.
- Over a detached garage, the whole building is a detached ADU, and the state only guarantees 16 feet in most locations. A garage plus a full living floor rarely fits in 16 feet.
That does not mean a detached two-story is impossible. It means it depends on your local ordinance going further than the state requires, and that is where the city or county you are in matters.
What Monterey-Area Jurisdictions Allow
City of Monterey. The city’s ADU code (section 38-112.6) mirrors the state numbers: attached ADUs up to 25 feet or the primary dwelling limit, with a two-story maximum; detached ADUs at 16 feet, or 18 feet near transit with up to 20 feet when the roof pitch matches the main house. It also includes a specific route for exactly this project: a second-story unit that would exceed the detached height limit can go through discretionary architectural review if the total building height is 25 feet or less and the parcel is in the R-3 zone. That is a discretionary process, not a ministerial one, so plan for more time and a design that will survive review.
Unincorporated Monterey County. The county’s published ADU materials give 16 feet for detached ADUs and say nothing specific about two-story or above-garage units. Treat a detached garage-plus-ADU in the county as something to confirm with planning before design starts, not something to assume. Coastal parcels add a Coastal Administrative Permit on top of the building permit.
Carmel, Pacific Grove and Pebble Beach. Each applies its own ordinance and, in Carmel’s case, its own design review culture. The state minimums still apply everywhere, which is why the attached route is usually the dependable one.
Rules change, and ordinances get amended to track new state law. Confirm the current version for your parcel before you commit to a scheme.
Can the Garage Underneath Carry a Second Story?
This is the question that ends more above-garage projects than height does. Most garages were never designed to hold up a dwelling.
- Foundations. A garage typically sits on a slab with shallow perimeter footings sized for a light roof. A floor of living space, its walls, its roof and its occupants add load those footings were not sized for. The usual result is new or enlarged footings, sometimes underpinning.
- Walls and the door opening. The wide garage door opening is the weak point. Carrying a floor over it means a properly sized header or beam and posts that bring that load to footings designed for it.
- Lateral strength. A tall building with a large opening on one face needs shear walls or a frame to resist earthquakes and wind. On the Central Coast that is a design driver, not a detail.
- The soil. On a hillside lot, or where the soil is unknown, the engineer may need a soils report before sizing footings. Our post on geotechnical reports for hillside lots explains when that applies.
None of this is unusual, and none of it is guesswork once a structural engineer has looked at the existing garage. But it is the reason that “we’ll just build on top of what’s there” is sometimes cheaper as “we’ll take the garage down and build a new two-story structure correctly.” Finding that out early is cheap. Finding it out at plan check is not.
Fire Separation Between the Garage and the ADU
A garage under a home is a fire risk the code takes seriously. The California Residential Code (section R302.6) sets minimum separations between a garage and living space:
- Where there are habitable rooms above the garage, the garage ceiling needs at least 5/8-inch Type X gypsum board.
- The walls and beams that hold up that ceiling need at least 1/2-inch gypsum board.
- Between the garage and the rest of the dwelling, the garage side gets at least 1/2-inch gypsum board.
Two more points follow from building a second dwelling unit. The separation between the ADU and the primary home, where they share a wall or floor, is governed by the code’s rules for two dwelling units in one building. And an ADU is not required to have fire sprinklers unless sprinklers are required for the primary residence (Government Code section 66314(d)(12)).
Stairs, Entry and Parking
An upstairs ADU needs its own way in. That usually means an exterior stair, which has to meet the code’s rise, run, width and guardrail requirements and still fit within the setbacks. On a narrow lot, the stair is often the hardest thing to place on the site plan.
Parking is the good news. Because you keep the garage, you keep its parking. And if your jurisdiction would otherwise want a space for the ADU itself, state law removes that requirement in many situations, including lots within half a mile of public transit.
Can an ADU Have Its Own Garage?
Yes. Nothing in state law prevents an ADU from having an attached garage, and some homeowners would rather build a new detached structure with a garage at ground level and the unit above than modify an old garage.
Two things to check. First, size: ADU limits are measured in interior livable space, and a garage is not livable space, but some local ordinances count accessory floor area differently. Second, height: a new garage-plus-ADU building on its own is still a detached ADU, so the height rules above apply to it exactly as they apply to a conversion.
Above the Garage, Conversion or Detached: Which One Fits?
| Option | Best when | Watch for |
|---|---|---|
| ADU over the garage | The lot is small, you need to keep the parking, and the garage is attached or the local code allows a taller detached building | Height limits on detached garages, structural upgrades, stair placement |
| Garage conversion to ADU | You can live without the garage and want the lowest-cost path to a legal unit | Losing storage and parking (though the city cannot require you to replace the parking) |
| New detached ADU | There is yard space and you want a single-story unit or full design freedom | Setbacks, the 16-foot height cap, utility runs |
If you are weighing a conversion that keeps the space part of the main house rather than creating a separate unit, our guide to converting an attached garage into living space covers that very different set of rules. For how big the unit can be, see the ADU size guide.
What the Drawings for an Above-Garage ADU Include
A permit set for this project is more involved than for a ground-floor unit, because it documents an existing structure and a new one at once:
- A site plan showing setbacks, the stair, height and parking
- Existing-condition drawings of the garage, so the engineer and plan checker can see what is being built on
- Floor plans, elevations and sections showing the overall height against the limit that applies
- Structural drawings and calculations from a licensed engineer: footings, beams over the door opening, lateral design
- Fire-separation details at the garage ceiling and any shared walls
- Title 24 energy compliance
Renovation Drafting & Design prepares ADU permit drawings across Monterey, Carmel, Pacific Grove, Pebble Beach and Carmel Valley, and coordinates the engineering. See our ADU design service or get in touch with your address, and we will tell you which height rule applies before you spend anything on design.
Codes and local ordinances change. Requirements described here reflect published state and local rules at the time of writing; confirm current rules for your parcel with the local building and planning department.
ADU Over Garage FAQs
Can you build an ADU above a garage in California?
Yes. State law allows ADUs that are attached, detached or within existing space, and an ADU above a garage fits within those categories. The limiting factor is usually height: an ADU above an attached garage is an attached ADU and the state guarantees up to 25 feet or the zone’s height limit, while an ADU above a detached garage is a detached ADU, where the state minimum is 16 feet in most locations.
How tall can an ADU above a detached garage be?
The state guarantees 16 feet for a detached ADU on a single-family lot, or 18 feet within half a mile of major transit, plus up to 2 feet for roof pitch. A two-story garage-plus-ADU usually needs more than that, so it depends on the local ordinance allowing extra height. The City of Monterey, for example, allows discretionary architectural review for second-story units up to 25 feet total in the R-3 zone.
Does my garage need to be reinforced to add an ADU on top?
Usually. Garage footings and the beam over the garage door are rarely sized for a second floor of living space. A structural engineer evaluates the existing foundation, the header over the door opening and lateral resistance to earthquakes, and new or enlarged footings are common.
What fire protection is required between a garage and an ADU above it?
Under California Residential Code section R302.6, the garage ceiling below habitable rooms needs at least 5/8-inch Type X gypsum board, and the structure supporting that ceiling needs at least 1/2-inch gypsum board. Fire sprinklers are not required in the ADU unless they are required for the primary residence.
Do I lose my parking if I build an ADU above the garage?
No. The garage stays, so its parking stays. State law also removes the requirement for an additional parking space for the ADU in many situations, including lots within half a mile of public transit.
Can an ADU have an attached garage?
Yes. State law does not prevent an ADU from having a garage. ADU size limits are measured in interior livable space, which does not include a garage, but some local ordinances count accessory floor area differently, so confirm how your jurisdiction measures it.