Microsoft and Group Health Cooperative, tenants of the Westlake Terry building, will have to vacate once Amazon is ready to move into the entire office complex.

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Amazon is kicking rival Microsoft out of one of South Lake Union’s major office complexes as the online retailer continues its expansion binge in Seattle.

Amazon signed a 12-year lease to occupy all 320,000 square feet of the Westlake Terry property, which consists of two adjacent buildings covering a full block between Harrison and Thomas streets. It can accommodate more than 1,000 employees.

Microsoft has leased a big chunk of the complex — the company’s only office presence in Seattle — since it opened last decade, housing about 350 employees at one time.

The other major tenant is Group Health Cooperative, a nonprofit health-care system which has its administrative headquarters at the six-story structure.

But the building wasn’t fully leased long-term. Its owner, Los Angeles-based Kilroy Realty, was looking to lock up a single tenant to fill the entire space, and said only Amazon was willing to do so.

The deal, announced Monday, means Microsoft and Group Health will have to leave when their current leases are up, then the interior of the two connected buildings will be reconfigured to accommodate a layout for one company. An exact timeline for the move isn’t clear, but it’s not believed to be imminent. Several restaurants on the ground floor won’t be affected.

Microsoft and Group Health did not immediately respond to questions Monday.

The Pacific Northwest’s two largest tech companies compete for talent and market share but rarely see their office needs conflict, given that they’re based on opposite sides of Lake Washington.

Microsoft has had other offices in Seattle before, including in Pioneer Square (outside of Amazonia) but the Westlake Terry building is its only active office site left in the city. It’s not clear if Microsoft will keep a Seattle office presence after the lease ends. The company had as many as 1,400 employees in the city last decade.

Amazon, on the other hand, has an insatiable thirst for Seattle real estate to house its rapidly expanding workforce.

Amazon is on track to occupy 12 million square feet in Seattle — enough to fill a skyscraper that would stretch more than a mile high. Just six weeks ago, the company leased the building directly to the north of its forthcoming digs in the Westlake Terry building, and already occupies offices to the south, east and west.

Paul Allen’s Vulcan Real Estate opened the Westlake Terry project in 2007 and joined with Group Health to sell the property to Kilroy for $170 million in 2013.

Kilroy declined to comment on Microsoft and Group Health vacating the building. Amazon said in a statement it was “looking forward to moving into another great office space.”