Whenever the county legislative authority deems it advisable and necessary for the public health and welfare of the inhabitants of the county to establish, purchase, acquire, and construct a system of sewerage and/or water, or make any additions and betterments thereto, or extensions thereof, the board shall adopt a sewerage and/or water general plan for a system of sewerage and/or water for all or a portion of the county as deemed necessary by the board. If the county has adopted a comprehensive plan for a physical development of the county pursuant to chapter
36.70 RCW and/or chapter
35.63 RCW, then the sewerage and/or water general plan shall be adopted as an element of that comprehensive plan pursuant to the applicable statute.