The total land area devoted to open space, exclusive of one-family and two-family lots, shall not be less than twenty percent of the gross land area included in the building lot. Such open space shall be maintained as grassed and landscaped areas, interior or exterior malls, pedestrian walks, and ornamental structures, when part of the landscaping theme. Open space shall not include structures or buildings, off-street parking areas, loading areas and access drives. Trees and shrubbery shall be provided at a minimum of one landscape unit per 3,000 square feet of required open space with one landscape unit consisting of two trees and six shrubs or three trees and three shrubs. There shall be a minimum of one landscape unit per building lot. Landscape units are in addition to screen plantings.