The big headquarters campus that Vulcan Real Estate is building for in Seattle's South Lake Union neighborhood could get even...

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The big headquarters campus that Vulcan Real Estate is building for in Seattle’s South Lake Union neighborhood could get even bigger.

Amazon has revealed in a federal regulatory filing that, in addition to 1.6 million square feet of office space already in the pipeline, it has options to lease an additional 500,000 square feet.

Vulcan government-affairs manager Lyn Tangen confirmed that Amazon’s option deal is with Vulcan. She wouldn’t discuss the location or timing of any potential new buildings.

Amazon spokesman Craig Berman declined to comment.

Amazon and Vulcan announced the headquarters deal in December. It called for Amazon to lease up to 1.6 million square feet in as many as 11 new Vulcan-built buildings along Terry Avenue North and Boren Avenue North between John and Mercer streets.

The first of those buildings is scheduled for completion in 2010.

The original announcement said Amazon had committed to lease about 800,000 square feet for at least 16 years and had options to lease the other 800,000 square feet.

The online retailer has since exercised its option on about 540,000 square feet, it revealed in a filing with the Securities and Exchange Commission in late July.

That filing also disclosed, for the first time, that Amazon has options to lease an additional 500,000 square feet on top of the original 1.6 million.

While lease rates for the campus buildings already in the works have been negotiated, the filing said, rent for any additional space would be “based on fair-market values at the time the options are exercised, subject to certain conditions.”

Steven Paget, chairman of the South Lake Union Friends & Neighbors Community Council, said he knew nothing of plans for an even larger Amazon campus. But he said he had heard that Amazon plans to move more employees to South Lake Union than it planned when the campus development was announced.

Amazon now leases space in several Seattle buildings, including its current headquarters at the Pacific Medical Center on Beacon Hill.

Vulcan owns several acres in South Lake Union for which it has not announced development plans, mostly north and west of the Amazon campus. Blocks between Mercer Street and Lake Union would be prime candidates for additional Amazon buildings, several commercial real-estate professionals said, but pending city plans to widen Mercer Street could complicate development.

In addition to the Amazon buildings, Vulcan has two other office projects in the works in South Lake Union.

The 2201 Westlake building, with 300,000 square feet, is scheduled to open next year. Microsoft is widely rumored to have a tentative deal to lease the entire building.

Vulcan also is working with city officials on the preliminary design for a 225,000-square-foot office building at Westlake Avenue North and Mercer that would incorporate parts of two historic auto showrooms now on the site.

Eric Pryne: 206-464-2231 or