The District of Columbia has the second-highest rental housing prices in the country behind Hawaii, according to the U.S. Department of Housing and Urban Development.

The average monthly rent for a two-bedroom unit in D.C. costs roughly $1,506, HUD said. Maryland ranks fifth with an average monthly rent of $1,291 for a two-bedroom. Virginia ranks ninth nationally, at $1,054 a month.

Altogether, the greater Washington region has the nation's 10th most expensive rents, according to the National Low Income Housing Coalition.

"Rent is at an all-time high," said Gregory Leisch, founder and CEO of Delta Associates, an Alexandria-based real estate firm. "We have tracked rents in the Washington, Baltimore and Philadelphia market for 18 years, and they are at the highest levels that we have ever seen."

Real estate experts attribute D.C.'s pricey rents to its low apartment vacancy rates, which average roughly 3.8 percent, far below the national average of 5.8 percent.

Only New York and Philadelphia have lower vacancy rates, according to Delta Associates.

D.C.-area rents are also being driven up by the growing number of single-person households. Only Atlanta has more people living alone, Census data show. Neighboring Arlington ranks seventh nationally in one-person households.

D.C. residents also earn more than most of the rest of the country, which allows property owners to push up prices.

The District's median annual income is $107,500, the highest in the U.S., followed by Maryland, at $91,917. Virginia's median income is $78,620.