Cal. Veh. Code § 4004.5
The owner of any motor vehicle subject to the registration provisions of this chapter, or exempted therefrom, may file with the department, at the time he registers or renews the registration of such motor vehicle, a signed statement that he resides in more than one county for a period of more than 30 days, or uses such vehicle in a county other than the county of his legal residence for business purposes. Such statement shall specify the address at which the owner resides in any county for more than 30 days, if any, or the length of time during which such owner uses the motor vehicle for business purposes in any county.
The department shall provide forms for the filing of such statements and shall take into consideration the length of time during which owners reside or use motor vehicles in counties, in accordance with the statements filed with the department, to determine the total number of vehicles registered or exempted from registration in any particular county.
Ca. Veh. Code § 4004.5