Correction: The article misstated the value of the deal. It was $6.5 billion, not $6.5 million, in cash and stock. This version has been updated.

Last week’s sale of Archstone, one of the country’s largest builders and managers of apartments, puts more than 22,000 units newly in the hands of Arlington-based AvalonBay Communities in the midst of a booming Washington apartment market.

Lehman Brothers led a move to take Archstone private in 2007, near the peak of the real estate market, but filed for bankruptcy in 2008 and has been liquidating its assets.

It agreed to sell Archstone, based in Englewood, Colo., for $6.5 billion in cash and stock. About 40 percent of the company will go to AvalonBay, a total of 66 apartment communities with 22,222 units. In addition, the company will also take control of six apartment projects that are under construction, which will add 1,666 more units.

The remainder of Archstone, including about 23,000 apartments, will go to Equity Residential, the Chicago-based firm co-founded by private-equity titan Sam Zell. The deal is expected to close in the first quarter of next year.

Timothy J. Naughton, AvalonBay’s president and chief executive, issued a statement saying the deal “accelerates our strategic growth vision of more deeply penetrating our core, high barrier-to-entry coastal markets.

“This is a rare opportunity to acquire a high quality portfolio of apartment communities concentrated in our markets, to better achieve our geographic portfolio allocation goals, to enhance operating efficiencies and to further advance our multi-brand strategy,” Naughton added.

Since the housing bubble burst, Washington has emerged as one of the strongest apartment development and investment markets in the country. During the 12 months ending in the second quarter of this year, developers in the Washington region added 11,068 apartment units, nearly double the number for the same period from 2011 to 2012, according to the Alexandria research firm Delta Associates.

AvalonBay has been in the middle of the boom. The firm bought into a delayed condo project in Tysons Corner, Park Crest, that includes a Harris Teeter, and completed the 354-unit Avalon Park Crest last year. It owns a second 5.5-acre site on Tyco Road nearby, where executives have discussed building two six-story buildings.

In Merrifield, AvalonBay is building 531 apartments above ground-floor retail as part of a 1.9-million-square-foot town center, the Mosaic District, near an Orange Line Metro station.

In the District, AvalonBay bought a site out of foreclosure on I Street NE, near the city’s booming H Street corridor, and is planning 140 high-end units with small floor plates, including one-bedroom units as small as 600 square feet, according to a conference call Naughton held with investors in October. That building will be called Ava H Street.

Of the properties that AvalonBay is acquiring from Archstone, 18 are in the Mid-Atlantic and total 5,311 units — giving the firm an even deeper foothold in the region.

But although the vacancy rate among Washington area apartments remains almost historically low and rents continue to rise, there are already signs that rent increases are slowing, and investors and banks appear to have taken notice. The price-per-unit for apartment sales in the region is down 5 percent so far this year from 2011 for high-rise properties and 13.1 percent for low-rise units, according to Delta Associates.