Open space and recreation area. At least 50% of the gross area of the site shall be preserved as permanent open space, free of buildings and parking areas and shall be landscaped or left in its natural state in accordance with plans approved by the Planning Board. Within such common open space areas, a total of not less than 300 square feet per dwelling unit shall be improved with common recreational facilities, such as swimming pools, tennis, basketball, volleyball and shuffleboard courts, playground equipment, etc., for the use of the residents of the premises and their guests, which facilities shall not be operated for profit.