Wat. Code, § 35500

A district may acquire, construct, operate, and furnish facilities and services, within or without the district, for the collection, treatment, and disposal of sewage, waste, and storm water and may contract with any persons, firms, public or private corporations or public agencies or other users concerning facilities and services for said purposes. The district may only furnish such services outside the district by means of facilities designed primarily to serve inside the district. Before a district may so furnish such services without the district and within the boundaries of another district or a municipality which has the same or similar powers, including a California water district, with respect to such facilities and services, it shall secure the consent of the governing body of such other district or municipality to do so. This section, to the extent it relates to service outside the district, shall only apply in the case of California water districts where two California water districts are contiguous to each other.