Ontario, CA Accessory Structures: Garage Conversions (2026)

010 of the Ontario Development Code governs converting an existing garage or accessory structure into an accessory dwelling unit. Under the statutory Class 1 ADU provisions (Gov. Code 66323), one ADU is allowed on a single-family lot within the existing space of the dwelling or an existing accessory structure, plus up to 150 additional square feet limited to accommodating ingress and egress, provided it has independent exterior access and side/rear setbacks sufficient for fire and safety as dictated by the building and fire codes. No setback is required for an existing accessory structure, or a structure rebuilt in the same location and dimensions, that is converted to an ADU.

When a garage, carport, covered parking structure, or uncovered parking space is demolished in conjunction with constructing an ADU or is converted to an ADU, those off-street parking spaces are not required to be replaced. A demolition permit for a detached garage that is to be replaced with an ADU is reviewed and issued together with the ADU application. ADUs created by garage conversion that result in removal of the garage door must provide a minimum 3-foot-wide foundation landscape planter for the full length of the former garage-door wall, fully landscaped, irrigated, and permanently maintained.

Conversions must still meet the ADU architectural requirements, including matching exterior materials, colors, and roof slope to the primary dwelling, an independent exterior entrance, minimum interior dimensions of 10 feet in every direction, and a 7-foot minimum interior wall height. On a lot with a multi-family dwelling, one or more ADUs may be created within non-livable space such as storage rooms, basements, attics, passageways, or garages, up to 25 percent of the existing units.