(a) To the extent that the incorporation will promote the public health, safety, and welfare, each county plan shall incorporate all or part of the subsidiary plans of each town, municipal corporation, sanitary district, privately owned facility, or local, State, or federal agency that has existing or planned development in that county.
(b) If the governing body of each county that is affected adopts a subsidiary plan for a multicounty area, the county may incorporate in its county plan all or part of the subsidiary plan.