The Planning Board is authorized, by grant of a special permit, to allow an increase in density of up to three times the density allowed under the conventional provisions of the Zoning Bylaw for a project located in the following zoning districts: R-80, R-60, R-40, Village Commercial and Commercial if the project contains the following minimum dry acreage: in the R-80 District, no less than 10 acres; in the R-60 District, no less than 7.5 acres; in the R-40 District, no less than five acres. A project approved under this section shall provide 30% or more of the proposed housing units as affordable housing units, either for sale or rental. The Planning Board may waive the requirements for lot size and lot line setbacks to accommodate the approved density and shall specify in its approval of the special permit the lot size, setback requirements it deems appropriate. Projects approved under the provision of this section shall be subject to all other appropriate provisions of Article XIX, Affordable Housing, and shall be subject to the provisions of Article XV, Site Plan Approval.