WASHINGTON — San Francisco-based Digital Realty Trust is set to buy more than 400 acres of land next to Dulles Airport from the Metropolitan Washington Airports Authority for $236.5 million.

The airports authority board is expected to approve the sale at its meeting Sept 17.

MWAA bought more than 850 acres of land between 2005 and 2007 to support plans for a fourth runway and support facilities. It will need just half that.

In 2017, the airports authority hired CBRE to market 424 acres of what is known as the Western Lands for sale or lease. It received multiple offers and negotiated with the highest bidder, Digital Realty.

Digital Realty signed a letter of intent to buy the land in May, including a $5 million deposit, and after its 60-day review period, will move forward. The sale will close by mid-October, pending board approval next week.

The Airprorts Authority will net an estimated $207 million from the sale, and will earmark the money to reduce the costs it includes when calculating rentals, fees and charges it assesses airlines operating at Dulles.

Digital Realty already owns and operates more than a dozen data centers in Northern Virginia. Last year, it acquired big District-based data center owner DuPont Fabros for $7.6 billion.

Northern Virginia leads the North American data center market, with more than 30 percent market share, according to JLL. And Loudoun County leads the region’s data center concentration.

Santa Clara, California-based Vantage Data Centers acquired dozens of acres in Ashburn, Virginia, last fall and is building a $1 billion data center campus with more than a million square feet.