Two years removed from her Housing Complex gig, my predecessor Lydia DePillis decided to prove she’s still got her D.C. real estate reporting chops. At the Washington Post‘s Storyline today, she has a great distillation of the central paradox confronting renters in the city: With cranes topping off new apartment buildings seemingly each week, why’s it so hard to find an affordable place to live? The answer comes in the form of another question: affordable to whom?

For the District’s high-earning young professionals without families to worry about, there’s no shortage of available apartments. The supply of “class A” apartments—-new buildings loaded up with amenities—-grew more than 70 percent between the end of 2010 and this summer, DePillis reports. Sure, the city’s population is growing, to the tune of 1,000 new residents a month, but high-end apartments are more than keeping pace.

The trouble is that not everyone can afford these apartments: The average class A rent in the Northwest quadrant is $2,648, according to Delta Associates. (The figure is for apartments of all sizes; studios and one-bedrooms are lower.) That actually represents a slight decrease from a year ago, given the expanding supply, but it doesn’t really matter to the majority of D.C. residents, who wouldn’t be able to afford these units even if prices dropped by 10 percent or more.

Then there are the “class B” apartments. These are older buildings with fewer amenities, and by definition their supply can’t grow much. As a result, their vacancy rate is low—-3 percent, compared to 4.5 percent for class A units—-and their rents are rising. In other words, the portion of the District’s housing supply that’s supposed to be more affordable is growing less affordable, while the portion that’s expanding is well out of the reach of the bulk of the populace.

Developers DePillis spoke with pointed to the thing they always point to: financing. It’s easier to get banks to make loans for high-end housing than for affordable or family units. This, of course, highlights the importance of the city’s role in steering development toward these apartment types that are more difficult to come by on the free market. The District’s inclusionary zoning law requires new residential developments to set aside around a tenth of their units for low-income residents, but the program has been slow to get off the ground. That places ever more significance on the city’s use of its publicly owned land, like a big vacant property near the Petworth Metro station that’s drawn pleas from neighbors alternately for affordable and market-rate housing. The wisdom of concentrating too much affordable housing in any one place is still up for debate, but as DePillis’ story makes clear, if the city doesn’t muster its resources to promote affordable housing, the market won’t do so on its own.

Photo by Darrow Montgomery