California Code, Elections Code - ELEC § 2034

A person domiciled in a house or apartment lying in more than one precinct shall be registered as domiciled in the precinct designated by the county elections official on the basis of the street address or other precinct the county elections official considers appropriate unless the person requests, either by letter or in person at the office of the county elections official, that he or she wishes to be domiciled for registration purposes in another precinct in which his or her house or apartment lies.  In order to fulfill the requirements of this section, the letter of request shall include the name, signature, and residence address of the requester.