Cal. Health & Safety Code § 6420

Whenever 25 persons in any county, or in two or more counties within the same natural watershed area, desire the formation of a sanitary district within the area, they may sign and present a petition to the board of supervisors of the county in which the greatest portion of the area of the proposed district is situated at the time of the filing of the petition. If the district is to be located in more than one county, the petition shall be signed by no fewer than 15 persons in the county in which the greatest portion of the district is located and by no fewer than 10 persons in each other county in which the district is located.