Any site plan approval granted to a property owner within the Residential Multi-Family Zone District which includes Affordable Units pursuant to the provisions of this chapter shall include provisions that shall permit the property owner to elect which units on site shall be designated as Affordable Units. If the site plan approval for Affordable Units is an amended site plan for premises already approved and developed as non-affordable units, the property owner, at the property owner's election, may designate the existing units or the newly-approved units as Affordable Units.