Alameda, CA Accessory Structures: Garage Conversions (2026)

18(c) sets specific standards for turning an existing accessory building, most often a detached or attached garage, into an accessory dwelling unit. Under subsection (c)(4)(b)(1), no side or rear setback is required at all when the ADU is created in an existing accessory structure or in a new structure built in the same location and at the same exterior width, length and height, up to 18 feet, as the structure it replaces. This removes the standard 4-foot setback that applies to new detached ADU construction, making a straight garage conversion far easier to permit than building new.

In exchange, subsection (c)(5)(c) imposes a design condition: an existing accessory building such as a detached garage that converts to an ADU must replace its garage door with the same exterior wall material, building color and window trim as the existing building, or as the primary dwelling if the garage being converted is attached to the house. This prevents a converted garage from reading as a garage with a window awkwardly punched into the door opening. The expansion allowance in subsection (c)(4)(e) lets a garage conversion add up to 150 square feet beyond the structure's existing physical dimensions, with any expansion beyond that limited to what is needed for ingress and egress.

Lot coverage from the converted structure still counts toward the 60% aggregate cap under subsection (c)(4)(d), though that cap does not apply if the ADU is built in the same location and dimensions as the existing structure it replaces.