When residential buildings or residential portions of mixed buildings are rehabilitated, the density regulations of the underlying districts shall not apply. In lieu thereof, there shall be not more than one room for each 230 square feet of gross floor area within the rehabilitated residential building or residential portion of a mixed building.

Furthermore, when a non-residential building, or portion thereof, is converted for residential use, the density regulations of the underlying districts shall not apply to that portion of the building containing dwelling units. In lieu thereof, there shall be not more than one room for each 230 square feet of gross floor area provided within the converted building or portion thereof.