Can I convert storage rooms or garages in my apartment building into ADUs?

Yes. Government Code section 66323(a)(3) requires cities to allow ADUs within portions of an existing multifamily building that are not used as livable space, such as storage rooms, boiler rooms, passageways, attics, basements, or garages. You can create at least one, and up to 25 percent of the number of existing units in the building, and these units are approved ministerially under state standards rather than local design rules.

What counts as multifamily

For state ADU law, a structure with two or more attached dwellings on one lot, such as a duplex or an apartment building, is a multifamily dwelling. Several detached houses on one lot are not. JADUs are not allowed on multifamily sites.

Other options on the same lot

• Up to eight detached ADUs on a lot with an existing multifamily building, not more than the number of existing units, or up to two on a lot with a proposed multifamily building, with four-foot side and rear setbacks.
• Conversion ADUs and detached ADUs can be combined.
• Cities cannot impose parking requirements on these units, and a leasing office can also be converted.

Building and fire code issues

• The new unit must meet building, health and safety, and fire standards for dwellings, including fire-resistive separation from neighboring units and code-compliant exits.
• Fire sprinklers are not required in the ADU unless they are required for the existing building, and adding the ADU cannot trigger sprinklers in the existing multifamily building. If the building is already sprinklered, plan on extending coverage.
• Accessibility standards may apply to converted space; HCD directs those questions to its Division of Codes and Standards.

Energy Code: multifamily rules apply

A Group R-2 apartment building is a multifamily building under the 2025 Energy Code. Converting unconditioned storage or garage space into a dwelling unit creates newly conditioned space, which follows the addition rules in Section 180.1. Under its exceptions, solar PV and battery storage are not required for additions, and new space heating serving an addition may be a heat pump or a gas system where local rules allow gas. New dwelling units that are additions must still have whole-dwelling mechanical ventilation based on their own floor area, plus local exhaust.

What it means for your MEP plans

• Electrical: a load calculation for the building service, a new meter position or subpanel for the unit, and coordination with the utility if the meter bank changes.
• Plumbing: routes for a full kitchen and bathroom through an existing building, often tying into existing stacks after checking their capacity.
• Mechanical: a separate heating and cooling system for the unit, kitchen and bath exhaust, and ventilation, with no return air shared with other units.
• Utility connections: unlike single-family conversions under 66323(a)(1), HCD's handbook indicates a new or separate connection may be required for these units, so ask your water and sewer providers early.

See the California ADU requirements guide, how many ADUs a lot can have, and electrical load calculations.

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