Vacaville, CA Accessory Structures: ADU Rules (2026)

A detached ADU built under the City's Class 1 statutory track is generally capped at 18 feet in height, rising to 20 feet on lots within a half mile of a major transit stop if the extra height matches the primary home's roof pitch, or up to 25 feet for an ADU stacked above a garage or attached to the primary dwelling. Under the locally regulated Class 2 track, an attached or detached ADU must be at least 150 square feet and no more than 1,200 square feet of interior livable space, with an attached unit further capped at 50 percent of the primary dwelling's floor area, and it must meet four-foot side and rear setbacks and a 20-foot front setback.

One paved parking space is required per ADU or bedroom, whichever is less, but that requirement is waived if the ADU sits within a half mile of public transit, in a historic district, converted from existing space, on a lot where street parking permits aren't offered to the occupant, or near an established car-share stop. Fire sprinklers are only required in an ADU if they're required in the primary residence, and building an ADU doesn't trigger a sprinkler retrofit for the existing home. A junior ADU, capped at 500 square feet within the walls of the primary home, has its own efficiency-kitchen and owner-occupancy requirements, and a deed restriction must be recorded before the City issues its certificate of occupancy.