The City of San Diego’s Development Services Department published Technical Bulletin RESD-3-4 in September 2025 to explain ADU sprinkler requirements when the primary residence is not sprinkler-protected. Its scenarios depend on whether the ADU is attached or detached, how the main building is classified, and whether the unit is part of a City density-bonus program. The bulletin is a City of San Diego source, not a rule for every California jurisdiction.
Check the primary dwelling and ADU type
The bulletin states that new residential units require sprinkler protection under the current California Building Standards Codes, while describing a state-law ADU exception when the qualifying primary dwelling is not sprinkler-protected. It also says that Affordable ADUs and Bonus ADUs using the City’s ADU Home Density Bonus Program must include an automatic fire sprinkler system.
For a detached ADU, the City says the exemption applies only when the primary dwelling is not sprinkler-protected and the ADU is classified as Group R-3. If the ADU is added to a duplex or a multiple-dwelling-unit building, the bulletin treats that entire building as the primary dwelling for this check.
For an attached ADU, the City says sprinklers are required when the primary dwelling is sprinkler-protected. When the primary dwelling is not protected, the bulletin lists specific attached designs that do not require sprinklers, including one ADU attached to a single-family home. Do not assume that every attached conversion or multi-unit configuration fits one of those cases.
Confirm the exact City scenario
Before relying on an exemption, identify the existing building, determine whether its sprinkler system protects the primary dwelling, confirm the proposed ADU’s occupancy classification, and check whether a density-bonus program applies. Ask the City reviewer to confirm the applicable bulletin scenario against the current code and permit scope. Even where the bulletin describes a sprinkler exception, it says the design must still meet the health and safety requirements of the building standards.
Use the City’s current RESD-3-4 technical bulletin and the project’s permit review. This guide does not select a sprinkler design or establish approval.