Is converting an attached garage to an ADU different from a detached garage?

Yes, in a few ways that matter. Under the 2025 Energy Code both are additions, because unconditioned garage space becomes conditioned living space, but an attached garage creates a second dwelling unit inside the same building as your home, while a detached garage becomes its own small house. That difference drives the fire separation, the heating and electrical layout, and whether a JADU is even an option.

What stays the same

• State law treats both as conversions. HCD's ADU Handbook allows ADUs within an attached garage or within a detached garage or other accessory structure under Government Code section 66323(a)(1). These conversion ADUs are not subject to local size, height, lot coverage, or design standards, but they must meet the building, health and safety, and fire codes for dwellings.
• Energy Code classification. The California Energy Commission (CEC) lists converting an existing attached or detached unconditioned garage into an ADU as an addition under Section 150.2(a). Solar PV is not required for additions, but the new unit needs whole-dwelling ventilation, kitchen and bath exhaust, and ECC-rater verification where the code calls for it.
• Utility connections. For ADUs built within existing space under 66323(a)(1), the handbook says a new or separate utility connection cannot be required unless the ADU is built at the same time as a new single-family home.

What changes with an attached garage

• Separation from your house. The shared wall, and any shared floor or ceiling, now separates two dwelling units and has to be fire-resistive. Every pipe, duct, wire, and box that passes through it must keep that protection intact.
• No shared forced-air system. The CEC's 2025 compliance manual notes that the California Mechanical Code does not allow return air from one dwelling unit to be discharged into another through a shared heating or cooling system, so the ADU needs its own system and thermostat.
• A JADU is possible. State law treats an attached garage as part of the single-family residence for JADU purposes, so a JADU of up to 500 square feet is an alternative to a full ADU.
• Your home's equipment may be in there. Attached garages often hold the house's main panel, water heater, or laundry, which must be relocated or kept reachable.

What changes with a detached garage

• Utilities travel farther. Power usually arrives through a trenched feeder to a subpanel with its own grounding, and water, sewer, and any gas lines cross the yard.
• Distance to property lines. Many detached garages sit close to a side or rear lot line. Conversions are exempt from the four-foot setback but must have setbacks sufficient for fire and safety, and walls very close to a lot line can carry fire-resistance and opening limits that affect where windows, exhaust fan terminations, dryer vents, and mini-split line sets can go.
• No JADU. JADUs are not allowed in detached accessory structures.
• Room to expand for access. An ADU converted from an accessory structure may add up to 150 square feet beyond the existing footprint, but only for ingress and egress.

What it means for your MEP plans

For an attached conversion, your plans need a stand-alone heating and cooling system with its own thermostat, protected penetrations at the shared wall, and a clear answer for any house equipment left in the garage. For a detached conversion, the focus shifts to the feeder and subpanel, trench routes for water, sewer, and power, and penetration locations that respect any rated walls near the lot line. In both cases the Title 24 report models the space as an addition, and the MEP sheets must match it. See our garage conversion MEP plans, the garage conversion guide, and fire separation between the ADU and the main house.

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