Where the subdivision contains sewers, sewage treatment plants, water supply systems, park areas, street trees or other physical facilities, necessary or desirable for the welfare of the area and which are of common use or benefit and which are of such character that, the municipality, county or other public agency does not desire to maintain them, then provisions shall be made by trust agreements, which are a part of the deed restrictions and which are acceptable to the Planning Commission for the proper and continuous maintenance and supervision of such facilities by the lot owners in the subdivision.