Fountain Valley, CA Accessory Structures: ADU Rules (2026)

Fountain Valley Municipal Code § 21.08.055 sets the objective standards for accessory dwelling units in the City of Fountain Valley. Every ADU needs a building permit, but the application is considered ministerially, without discretionary review or a hearing, and the city must approve or deny it within 60 days of a completed application. Silence past 60 days means the application is deemed approved. A denial must come with a written list of defects and how to fix them.

Subsection (b) covers new construction. A new ADU must be at least 150 sq. ft. A detached unit can be up to 850 sq. ft., or 1,000 sq. ft. if it has more than one bedroom. An attached unit can be up to 50% of the primary residence, though that cannot force a unit below 800 sq. ft. A garage space designated for an ADU cannot exceed 200 sq. ft. A detached ADU over 800 sq. ft. must sit at least 10 ft from the main dwelling, and an attached ADU must share a wall of at least five feet, or half the ADU width if larger. The side and rear setback is no more than four feet.

Height for a detached ADU is a single story at 16 ft. Within one-half mile walking distance of a major transit stop or high-quality transit corridor the limit is 18 ft, plus two feet to match the roof pitch of the primary home. An attached ADU can rise two stories and 25 ft, or the zone limit if lower.

A parcel with one single-family dwelling can hold three units (home, ADU, junior ADU), or four with a detached new construction ADU of at most 800 sq. ft. and four-foot setbacks. The unit needs independent exterior access, a maximum of two entrances, clerestory or frosted windows within five feet of a property line, and exterior materials matching the home. Parking is one space per ADU or per bedroom, whichever is less, and can be tandem on a driveway. Rentals must exceed 30 days, and ADUs created after January 1, 2025 must be owner occupied, with a recorded covenant.