California Code, Government Code - GOV § 56741

Territory may not be annexed to a city unless it is located in the same county. Unless otherwise provided in this division, territory may not be annexed to a city unless it is contiguous to the city at the time the proposal is initiated pursuant to this part. Territory incorporated as a city shall be located within one county and, except as otherwise provided in Section 56742, shall be contiguous with all other territory being incorporated as a city.