WASHINGTON—The construction cranes that have crowded this city's skyline for the past few years were symbols of its red-hot apartment market. But now, the sight of them could be an ominous sign of overbuilding.

This year alone, developers are expected to deliver more than 15,000 new apartment units in the greater metro area, with 11,000 more expected in 2014. That is well above the more than 6,000 units delivered, on average, each year during the prior decade, according to research firm Delta Associates.

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