While President Trump talks repeatedly about fixing America’s inner cities, it’s a good bet that in the coming years, New York and other large metropolitan areas will need to be more self-reliant in solving pressing problems, especially low-income housing.

After all, many big cities face a triple threat: Mr. Trump wants to cut funding to sanctuary cities; his nominee to run the Department of Housing and Urban Development, Ben Carson, is unlikely to be a strong and creative leader; and the Republican Congress is eager to chip away at federal housing programs. In response, cities need local financing initiatives that make up for the coming reduction in federal assistance.

Fortunately, there’s an already tested alternative: an annual luxury housing tax, levied on new high-end condos and rentals, which would feed a self-sustaining fund dedicated to develop truly affordable units.

While no city has such a plan in place, this strategy has been tried right here in New York. The city has already channeled approximately $1 billion from luxury development for affordable housing into communities like Harlem and the South Bronx.