Usable open space. In all PDDs, at least 20% of the land shall be set aside as permanent usable open space, for the use of the PDD residents, or for all PDD users, or for the community. The required open space shall be conveyed to the Conservation Commission or to a nonprofit conservation organization, or to a corporation or trust representing persons responsible for the PDD, and shall be protected by a conservation restriction as required by MGL c. 40A, § 9 for common open space in cluster developments. A covenant shall be placed on the land such that no part of the PDD can be built, sold or occupied until such time as a satisfactory written agreement has been executed for protection of the open space.