Multifamily dwellings may be developed at a density not to exceed six dwelling units per gross acre, provided that the number of bedrooms, calculated in accordance with the definitions in this chapter for efficiency, one-bedroom, two-bedroom and three-bedroom apartment units, does not exceed nine bedrooms per gross acre. Efficiency units shall be considered to contain one bedroom. The applicant shall submit detailed floor plans for each unit type proposed, and the site plan and building plan shall specify the location of each unit type within the proposed buildings. Any change in the floor plan of a dwelling unit type or the location of dwelling unit types after the approval of a site plan shall require the approval of the Planning Board prior to issuance of a building permit.