The California Department of Fair Employment and Housing (DFEH) has issued an "FAQ for Employers" on transgender rights in the workplace. Among other things, the FAQ provides:
Employers should not ask questions designed to detect a person’s sexual orientation or gender identity or questions about an employee's plans to have surgery.
If an employer has a dress code, it should apply it a non-discriminatory manner. For example, an employee who identifies as a female should be held to the same standards as other females.
Employers should allow employees to use restrooms that correspond with their gender identities. Where possible, employers should provide private restrooms for use by any employee who wants greater privacy, including those who do not want to share facilities with transgender coworkers.
The FAQ is available here.