As 2012 ended, two of the biggest owners of apartment properties in the D.C. area teed up a deal to become even bigger in 2013.

They agreed to acquire the holdings of Archstone for $6.5 billion, which controls assets that had been caught up in the collapse of Archstone’s majority owner, Lehman Bros., in 2008.

Assuming the deal closes, the transaction will increase Equity Residential and AvalonBay’s holdings across the nation on aggregate by 27 percent, and even more so in the Washington area, where the number of units housing area residents will swell by 65 percent.

About one of every six apartment units owned by Equity will be located in the D.C. area, increasing its exposure here by three percentage points, and one in seven units owned by Avalon Bay will be in the region, a four-percentage-point increase in exposure to the nation’s capital.

Indeed, by increasing their slice of the D.C. region’s apartment inventory, the two apartment behemoths will control nearly 18 percent of the high-quality apartment units in the metropolitan area.

No doubt one of the most attractive components of the acquisition was the prime location of Archstone’s holdings. Almost three-quarters of the apartments trading hands are located in largely developed areas inside the Beltway that have proven to be attractive to young professionals.

Either Equity or AvalonBay will raise signs at nine properties for a total of 6,000 units along Metrorail’s Orange and Blue Lines in Northern Virginia. The deal also gives the landlords access to units with high price tags in tough-to-build-in Northwest D.C. neighborhoods along the Connecticut Avenue corridor — Dupont Circle up to Van Ness — where they will acquire five properties totaling 3,900 units.

For real estate investment trusts and other major investors looking to beef up their inventories of high-end units in excellent locations in short order, it looks like buying existing properties is just as good an option, or better, than building new. And while control and management will shift for those who currently occupy the apartments, the fate of those properties, which has been up in the air for more than four years, has finally been resolved.

Erica Champion is a senior real estate economist for CoStar Group in Washington.