From the ICC:
A building code is a collection of laws regulations, ordinances or other statutory requirements adopted by a government legislative authority involved with the physical structure and healthful conditions for occupants of buildings. The purpose of a building code is to establish the minimum acceptable requirements necessary for protecting the public health, safety and welfare in the built environment. These minimum requirements are based on natural laws, on properties of materials, and on the inherent hazards of climate, geology and the intended use of a structure (or its “occupancy”). The primary application of a building code is to regulate new or proposed construction. Building codes only apply to an existing building if the building undergoes reconstruction, rehabilitation or alteration, or if the occupancy of the existing building changes to a new occupancy as defined by the building code.
The 2016 California Building Standards Code (Title 24, California Code of Regulations) are effective as of January 1, 2017. Consult the CBSC site for any additional supplements or errata, as well as the dates in which they went into effect.