In furtherance of the purpose of providing affordable shelter for all residents of the Commonwealth, the governing body of any county where the urban county executive form of government or the county manager plan of government is in effect, the Counties of Albemarle and Loudoun, and the Cities of Alexandria, Charlottesville, Fairfax, and Falls Church may by amendment to the zoning ordinances of such locality provide for an affordable housing dwelling unit program. The program shall address housing needs, promote a full range of housing choices, and encourage the construction and continued existence of moderately priced housing by providing for optional increases in density in order to reduce land costs for such moderately priced housing. Any project that is subject to an affordable housing dwelling unit program adopted pursuant to this section shall not be subject to an additional requirement outside of such program to contribute to a county or city housing fund. Any local ordinance of any other locality providing optional increases in density for provision of low and moderate income housing adopted before December 31, 1988, shall continue in full force and effect.