Paul Krugman writes in the New York Times about solutions to a familiar situation — high housing costs in a desirable city, with prices out of the reach of ordinary working people.

The result, predictably, is that the urban renaissance is very much a class-based story. Upper-income Americans are moving into high-density areas, where they can benefit from city amenities; lower-income families are moving out of such areas, presumably because they can’t afford the real estate.

You may be tempted to say, so what else is new? Urban life has become desirable again, urban dwellings are in limited supply, so wouldn’t you expect the affluent to outbid the rest and move in? Why aren’t urban apartments like beachfront lots, which also tend to be occupied by the rich? . . . .

. . .  But living in the city isn’t like living on the beach, because the shortage of urban dwellings is mainly artificial. Our big cities, even New York, could comfortably hold quite a few more families than they do.

One obvious solution is to increase density. But, says Krugman

The question is, how can higher density be sold politically? The answer, surely, is to package a loosening of building restrictions with other measures. Which is why what’s happening in New York is so interesting.

In brief, Mayor Bill de Blasio has pushed through a program that would selectively loosen rules on density, height, and parking as long as developers include affordable and senior housing. The idea is, in effect, to accommodate the rising demand of affluent families for an urban lifestyle, but to harness that demand on behalf of making the city affordable for lower-income families too.