Can I build an ADU above my garage in California?

Yes. HCD's ADU Handbook lists an ADU built on top of an attached garage as an attached ADU, and a unit over a detached garage is generally a detached ADU; either way you are adding new space rather than converting existing space, so local height and other objective standards apply. For the 2025 Energy Code, the CEC treats a second story built over an existing garage as an addition, so solar PV is not required, but the new unit must meet ventilation and verification requirements.

Height is usually the first hurdle

• Over a detached garage: state law sets the lowest height limit a city may impose on a detached ADU at 16 feet; 18 feet within a half mile of a major transit stop or high-quality transit corridor, plus up to 2 more feet to match your house's roof pitch; and 18 feet on a lot with a multistory multifamily building. A second story often needs more than 16 feet, so check what your city's ordinance allows.
• Over an attached garage: an attached ADU may be built to 25 feet or the height limit for your house, whichever is lower.
• Not a conversion: conversion ADUs are limited to the existing structure plus up to 150 square feet for ingress and egress, so a new floor on top follows the regular ADU rules instead.

Fire separation from the garage below

The garage ceiling under the ADU becomes a dwelling-garage separation. The California Residential Code calls for Type X gypsum board on that ceiling where habitable rooms are above a garage, along with protection of the structure that supports it. If the ADU sits over an attached garage, it also needs separation from your house as a separate dwelling unit.

Energy Code details

• The ADU floor sits over an unconditioned garage, so it must be insulated as a floor over unconditioned space.
• As a new dwelling unit added to an existing building, the ADU needs whole-dwelling mechanical ventilation based on its own floor area under Section 150.2(a)1C, plus kitchen and bath exhaust.
• PV is not required for additions (Exception 6 to Section 150.2(a)).
• Keep the air handler and ducts inside the ADU's conditioned space rather than in the garage.

What it means for your MEP plans

• Plumbing: Drains and vents from the upstairs kitchen and bathroom must drop through the garage to reach the sewer. Pipes passing through the rated garage ceiling need protected penetrations, and cleanouts must stay accessible.
• Mechanical: A ductless heat pump inside the ADU avoids running ducts through the garage. If ducts must pass through the garage, they must be sheet metal or another approved material with no openings into the garage. Plan the outdoor unit and line-set route early.
• Electrical: The ADU usually gets its own subpanel upstairs, fed from the main service, and the load calculation must show the service can carry both units. Stair lighting and smoke and CO alarms are part of the layout.
• Structure first: An existing garage foundation and walls may not be designed to carry a second floor. The structural solution often decides where vertical chases for pipes and ducts can go, so coordinate before finalizing the MEP layout.

See garage conversion MEP plans, the California ADU requirements guide, and electrical load calculations.

Sources: hcd.ca.gov, energy.ca.gov, energy.ca.gov, energy.ca.gov.