2019 California Standards for Accessible Design Guide (effective January 1, 2020 with July 1, 2021 amendments)

11B-608.7 Thresholds.

Thresholds in roll-in type shower compartments shall be ½ inch (12.7 mm) high maximum in accordance with Section 11B-303. In transfer type shower compartments, thresholds ½ inch (12.7 mm) high maximum shall be beveled, rounded, or vertical.

Exception: A threshold 2 inches (51 mm) high maximum shall be permitted in transfer type shower compartments in existing facilities where provision of a ½ inch (12.7 mm) high threshold would disturb the structural reinforcement of the floor slab.