Even owner-operated businesses with no employees must comply with California's workplace smoking laws. That is also true where the only workers are volunteers or independent contractors.
An employer may not knowingly or intentionally permit smoking in any enclosed workplace. That includes covered parking lots, lobbies, lounges, waiting areas, elevators, stairwells, and restrooms that are a structural part of the building.
Employers must also take reasonable steps to prevent nonemployees from smoking.
Yes, California's ban on workplace smoking also includes vaping and marijuana.
In California, smoking areas must be in a non-work area that is not enclosed. Employers may not offer "break rooms" for smokers.
Most California employers are required to post "No Smoking" signage at each workplace entrance.
In the few California workplaces where smoking is allowed (see the exceptions above), employers must post a sign stating that "smoking is only allowed in designated areas."
An employer may not discharge or discriminate against an employee for engaging in lawful activity during nonwork hours away from the employer's premises.
If you want to go right to the source and look up California law on workplace smoking laws -- or if you're writing a letter to your employer or employee and want to cite the applicable law -- the relevant statute(s) can be found at Cal. Lab. Code Section 6404.5.