Social media giant Facebook is growing its presence in Seattle in a big way, leasing a pair of planned office buildings in the South Lake Union neighborhood, right in Amazon’s backyard.

The two structures, known as the Arbor Blocks, are six stories each and total 384,000 square feet, along with 4,100 square feet of retail along Eighth Avenue North, between Thomas and Harrison streets. When they are done in the third quarter of 2018, the new buildings will nearly double Facebook’s footprint in Seattle.

Construction will begin on the $246 million project, developed by Paul Allen’s Vulcan Real Estate, by the end of the year. Designed by Graphite Design Group and to be built by Lease Crutcher Lewis, the project had been previously linked to Amazon.

“We’re excited to work with Facebook to help them expand their footprint in South Lake Union,” Vulcan Vice President of Real Estate Ada M. Healey said in a statement. “We designed the Arbor Blocks with technology tenants in mind and Facebook is an ideal tenant to kick-off the project.”

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The move is the latest sign of the seemingly insatiable appetite for expansion by Silicon Valley companies with outposts in Seattle. More than 80 companies from around the world have set up shop here in the region, and many of them have grown to become big parts of the local tech ecosystem.

Google last year also inked a huge lease from Vulcan, just a couple blocks from Facebook’s new buildings. The first phase of the four-building campus could start early next year and open in the fourth quarter of 2018.

Facebook’s expansion comes just a few months after the company moved its Seattle engineering center into a new office building on nearby Dexter Avenue. It has room for up to 2,000 people, but apparently that wasn’t enough.

Facebook has been on the hunt for more space recently, so this deal isn’t a huge surprise. It was previously linked to a project near Dexter Station at 1101 Westlake Ave. N. The developer of the Dexter Station project earlier this year filed plans for an expansion that could give the social network even more room, if it needs the additional space.

Vulcan was instrumental in transforming the South Lake Union neighborhood into one of the hottest tech hubs in the world. Vulcan’s deal with Amazon in 2007 to construct an 11-building headquarters in the neighborhood that helped kickstart the current wave of construction there. Amazon has since expanded in the neighborhood and companies from a variety of industries now call South Lake Union home.

But all that growth brings with it a crush of people and cars. For decades a challenging stretch of road, the so-called Mercer Mess is getting stressed even further by the hoards of people working in South Lake Union. Amazon already brings in thousands of people per day, Facebook’s Dexter Station office can hold up to 2,000, and the incoming Google campus has room for up to 4,000 people.

The city has been working to relieve the pressure by adding more lanes to the busy corridor, but studies show it hasn’t helped much. Developers and the companies that bring the people to the neighborhood are doing their best to take some cars off the road. As part of the Arbor Blocks project, Vulcan will build a woonerf, which is a Dutch term that means living yard, along part of Eighth Avenue. The concept is designed to make the area more pedestrian-friendly and encourage walking and biking by paving wider walkways and creating a blurred line between street and sidewalk, similar to Bell Street downtown.

Amazon recently started its own shuttle program, similar to the Microsoft Connector. It is designed to take employees from Eastside cities like Bellevue and Redmond to Amazon’s Seattle campus.

Here is the news release from Vulcan on the deal: