Each planning agency shall prepare a comprehensive plan for the orderly physical development of the county, or any portion thereof, and may include any land outside its boundaries which, in the judgment of the planning agency, relates to planning for the county. The plan shall be referred to as the comprehensive plan, and, after hearings by the commission and approval by motion of the board, shall be certified as the comprehensive plan. Amendments or additions to the comprehensive plan shall be similarly processed and certified.
Any comprehensive plan adopted for a portion of a county shall not be deemed invalid on the ground that the remainder of the county is not yet covered by a comprehensive plan. *This 1973 amendatory act shall also apply to comprehensive plans adopted for portions of a county prior to April 24, 1973.