The type of housing set-aside proposed in the bill is known as "inclusionary zoning" and has been used in dozens of other cities to help create affordable housing in overheated markets, like San Francisco, Boston and New York. The challenge for Philadelphia is that the city's boom has been relatively limited geographically, and the city still has vast areas where house prices are still low by national standards. Because so much of Philadelphia's housing stock is old and needs repair, some housing advocates said the city would be better off creating an impact fee, rather than mandate inclusionary zoning. That fee would go into the city's Housing Trust Fund, and would help support grants and loans for repairs. "The most affordable house is the one that people are already living in," Fadullon said.