Before granting such permission the legislative body of the county or city shall find that the public interest, convenience, and necessity require that the work or improvement be used, that water or gas is necessary therefor, and that a certain, named, district, public corporation, mutual company, public utility company, private corporation, or individual can most conveniently furnish water or gas. Thereupon the county or city may grant to such district, public corporation, mutual company, public utility company, private corporation, or individual the right to attach his lines or system to the work or improvement, and to furnish water or gas through the work or improvement to all lands which were assessed to pay the costs and expenses of constructing the work or improvement.