The approach may be new to Philadelphia, but the nation's reviving cities have been turning to inclusionary zoning as a way to cushion the impact of skyrocketing rents. Cities like San Francisco and Boston, with stratospheric housing costs, have even moved to make inclusionary zoning mandatory for all large apartment projects. New York just approved set-asides of 20 percent to 30 percent. At a time when the federal government has virtually cut funding to city housing authorities, inclusionary zoning is one of the few means left to pay for affordable housing.