Upon obtaining a special use permit according to the provisions of § 190-37, the requirements concerning lot area, frontage, side yards, and coverage may be modified for a development of not less than three acres, provided that the maximum density of the overall development is not greater than that normally allowable in the district in which it is located. Such land as may be made available under this section shall be devoted to common purposes for all residents, including, but not limited to, a park, playground or garden for the use of surrounding residents.