Alameda, CA Accessory Structures: ADU Rules (2026)

26. An ADU counts as an accessory use, not toward density limits, and cannot be treated as a new residential use for growth-control ordinances. Single-family lots may add one ADU plus one junior ADU; multifamily lots may add ADUs within existing building space without a numeric cap, plus up to two additional detached units. Size is capped at 1,200 square feet, though the code protects an 800-square-foot ADU at 16 feet tall with 4-foot side and rear setbacks regardless of other limits. A detached ADU built as new construction tops out at 18 feet, rising to 20 feet if its roof pitch matches the primary dwelling's; on lots in a FEMA special flood hazard area, the Building Official and City Engineer can allow additional height to clear the base flood elevation.

Standard setbacks from side and rear lot lines are 4 feet for a new detached unit, but the setback drops to zero feet on qualifying lots where the ADU sits 75 feet or more from the front property line, uses one-hour fire-resistive construction within 3 feet of the line, stays at or under 16 feet tall, and covers no more than 60% of the required rear yard. Lot coverage from all buildings on the parcel is capped at 60%. 2.