All of which has some people wondering: Is Philadelphia headed for an apartment glut?

The rental craze is happening all over the country. Vacancy rates for rentals are the lowest they’ve been in more than a decade, while the rate of homeownership has fallen to 65

percent—with a particularly striking decline among people between the ages of 25 and 34. Unemployed or underemployed and saddled with debt, the millennials aren’t taking chances when it comes to housing: Between 2006 and 2011, renters in that age group increased by more than one million.

Still, do these national patterns justify Philadelphia’s rental boom? Rental properties under construction here as of October 2013 totaled about 4,000 units. That number isn’t excessive for other cities—but it’s certainly robust for Philly.

“There is a little too much—though not a flood,” says realtor Allan Domb, president of the Greater Philadelphia Association of Realtors. “I think we’re going to have a short-term problem. There’s a bit of musical chairs going on.” Domb points out that five to 10 years ago, everyone was building high-rise condos in Center City. In recent years, money became available to build rental housing instead. But real estate is, as he puts it, a pendulum field. “What will happen is that the rental buildings will be converted to condominiums,” he predicts. “Within 12 to 24 months, the units will be absorbed.” In the meantime, Domb wouldn’t recommend that anyone build more rentals.

For the moment, apartment occupancy is extremely high—roughly 96 percent in Philadelphia. And fortunately for those new buildings opening up, more renters are on their way, including empty nesters coming back to town. Is the city of rowhomes being transformed into a city of apartments right before our eyes? We should have a more definitive answer in the next 24 months.