When the comprehensive plan containing the mandatory subjects as set forth in RCW 36.70.330 shall have been approved by motion by the board and certified, it may thereafter be progressively amplified and augmented in scope by expanding and increasing the general provisions and proposals for all or any one of the required elements set forth in RCW 36.70.330 and by adding provisions and proposals for the optional elements set forth in RCW 36.70.350. The comprehensive plan may also be amplified and augmented in scope by progressively including more completely planned areas consisting of natural homogeneous communities, distinctive geographic areas, or other types of districts having unified interests within the total area of the county. In no case shall the comprehensive plan, whether in its entirety or area by area or subject by subject be considered to be other than in such form as to serve as a guide to the later development and adoption of official controls.