Multifamily dwellings limited to occupancy by families or individuals with the head of the household being 52 years of age or older may be developed at a density not to exceed 11 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 15 bedrooms per gross acre, and the number of off-street parking spaces shall not be less than 1 1/2 spaces per dwelling unit. 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 plans 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 the issuance of a building permit.