San Diego Information Bulletin 400 distinguishes conversion, same-size reconstruction, additions, and new construction. The City’s 2025 Ordinance O-21989 adds setback and height branches that depend on structure height, fire-hazard designation, adjacent use, zoning, and Coastal Overlay status. Identify those facts before choosing a rule.
Conversion and floor area
The bulletin lists a 150-square-foot minimum and a 1,200-square-foot maximum gross floor area for attached or detached ADUs. It says the maximum does not apply to a conversion of an existing accessory structure or part of an existing single-dwelling unit. Other requirements remain. ADU floor area generally counts toward premises GFA and base-zone FAR, subject to bulletin-listed 800-square-foot exceptions in specified existing- or proposed-dwelling contexts. A conversion may use existing setbacks; demolition and reconstruction may retain them only when the replacement is in the same location and has the same dimensions. New construction or an addition follows a separate setback route.
2025 fire-zone and height branches
Under Ordinance O-21989, an ADU in a High or Very High Fire Hazard Severity Zone has a four-foot minimum interior side and rear setback. Outside those zones, a structure 16 feet high or less has no minimum side or rear setback under this zoning provision. For a structure over 16 feet outside those zones, no minimum applies unless the property line abuts residentially zoned or exclusively residentially developed land; there, the minimum is the lesser of four feet or the base-zone setback. The Fire Code Official may require a greater distance.
In a single-dwelling zone that does not allow multifamily development, an ADU attached to an existing accessory structure is limited to two stories. Other ADUs, and ADUs in multifamily zones, follow the underlying base- and overlay-zone maximum structure height. The City’s regulatory update says the 2025 amendments took effect August 22, 2025 outside the Coastal Overlay Zone; the Coastal provisions remain conditional on the stated Coastal Commission certification.
Confirm which rule applies
Check the current municipal code, base and overlay zones, project type, structure height, neighboring use, mapped fire-hazard status, and application date. Side and rear encroachments remain subject to California Building Standards Code requirements. A zoning setback exemption does not establish fire separation or building-code compliance. Ask the City and Fire Code Official about a parcel-specific condition. The 2025 ordinance controls where it updates the older bulletin; Coastal effectiveness must be verified.
Use the broader setback and separation guide and existing-structure relief guide to distinguish a dimensional rule from a conversion exception or discretionary relief.
