Cal. Health and Safety Code § 4830

Territory, whether situated within the same or another county and whether incorporated or unincorporated, which is not included in any other county sanitation district or other district formed for similar purposes, may be annexed to a county sanitation district if the directors find and determine that the additional territory will be benefited by annexation. Notwithstanding the provisions of the preceding sentence, territory, whether situated within the same or another county and whether incorporated or unincorporated, which is included in any district, which district is not, at the time of the proposed annexation, performing similar services in the area proposed to be annexed, may be annexed to a county sanitation district if the board of directors finds and determines that the additional territory will be benefited by the annexation. The land proposed to be annexed need not consist of contiguous parcels nor be contiguous with the county sanitation district when such land is within the same county. When land proposed to be annexed is not within the same county such land shall be contiguous to the district.