is buying its 11-building South Lake Union headquarters complex from Vulcan Real Estate for $1.16 billion. The giant office deal could signal that Amazon sees no end to growth. The state and city should get a financial windfall.

Share story is becoming its own landlord.

The burgeoning online retailer announced Friday that it is buying its 11-building South Lake Union headquarters complex from developer Vulcan Real Estate.

The price tag: $1.16 billion.

That breaks down to $644 per square foot for the 1.8 million-square-foot complex, making the sale arguably the richest office deal in Seattle’s history.

Amazon and Vulcan said they expect the sale to close before the end of the year.

Vulcan, owned by Microsoft co-founder Paul Allen, began building the complex in early 2008 after Amazon signed leases extending well into the 2020s. The last building is nearly complete.

Vulcan put the complex up for sale in late August, saying it wanted to rebalance its investment portfolio and free up capital for more development in South Lake Union.

Vice President Ada Healey said at the time that Amazon might be a potential buyer. But Healey said Friday that Vulcan didn’t have any indication then that Amazon was interested.

“We did not necessarily expect this outcome,” she said. “We did not know Amazon was going to be the buyer.”

If Amazon had not made an offer for all 11 buildings, Healey said, Vulcan probably would have sold the nine new and two renovated historic structures to as many as six separate buyers.

“Seattle ought to feel really good about an employer of Amazon’s magnitude making a commitment like this,” she said.

She wouldn’t discuss terms of the deal or other bidders.

The $1.16 billion price tag is about 65 percent more than the $700 million Vulcan reportedly paid to design and build the complex.

Amazon should have no trouble paying for the buildings. It held $5 billion in cash and marketable securities at the end of June.

The company did not return phone calls or emails seeking comment Friday.

Jeff Bezos, Amazon’s founder and CEO, has suggested that as the company grows and hires more employees, renting office space no longer makes economic sense.

“At that scale, renting that capacity would not be easy. In fact, it would be impossible,” Bezos said in May at Amazon’s annual shareholder meeting.

Amazon projected five years ago that it would pay $1.5 billion in rent, operating expenses and tenant improvements during the lease terms of the South Lake Union buildings.

“They’re in control of their destiny if they own their own buildings,” said Craig Hill, senior vice president with brokerage NAI Puget Sound Properties. “It’s a good investment — that neighborhood is only going to get better.”

Earlier this year Amazon announced plans to buy three Denny Triangle blocks from Seattle’s Clise Properties, and said it would build a 3.3 million-square-foot high-rise office campus there.

That prompted some to speculate that Amazon might abandon South Lake Union when its leases there expire years from now, said Oscar Oliveira, managing director at brokerage Broderick Group.

The sale should put that to rest, he said: “This takes a big question out of the market.”

In addition to the South Lake Union headquarters complex and the three-block Denny Triangle high-rise campus, Amazon also leases or has committed to lease 1 million more square feet in the two neighborhoods — and reportedly is in the market for still more.

“Each year they seem to expand more than anybody thought was possible,” Oliveira said.

Amazon does not disclose the size of its local workforce, but its online jobs board lists more than 2,000 openings in Seattle, and real-estate experts estimate that its South Lake Union campus has room for at least 9,000 employees.

Worldwide, Amazon reported 69,100 employees in late June, an increase of 3,500 from three months earlier.

Well-leased, top-end downtown Seattle office buildings have been selling for top dollar to institutional investors lately. The most lucrative were the Russell Investments Center, which sold earlier this year for $550 per square foot, and 818 Stewart, which fetched $558 last year.

At $644, the Amazon deal tops both by a wide margin.

Looking farther back, it is second only to the $665 per square foot that two smaller office properties near Pike Place Market fetched in 2007.

But real-estate insiders say that’s not a fair comparison, because the sale of the Market buildings was more than an office transaction: Much of their value was in a large parking garage built to accommodate not just office tenants but Market visitors.

Washington state and Seattle should benefit financially from Amazon’s purchase. Upon closing, Vulcan would pay about $14.8 million in real-estate excise tax to the state and another $5.8 million to the city.

Amazon announced the real-estate deal after Friday’s close on Wall Street. Shares of Amazon ended the regular trading session down $1.96, less than 1 percent, at $258.51. The stock has been trading near the upper end of a 52-week range of between $166.97 and $264.11.

The deal fits a pattern of Amazon spending heavily to set itself up for the long term. In July, the company posted a narrow second-quarter profit of $7 million amid new expenditures on distribution centers and digital-media content. Sales rose 29 percent from a year ago to $12.83 billion, cheering Wall Street.

“There doesn’t seem to be a tremendous amount of concern about Amazon’s bottom line,” said Needham & Co. analyst Kerry Rice.

Despite the steep price tag, Amazon’s move toward ownership could signal to investors that it sees no end in growth.

“You’d have to be pretty confident about the future to plunk down what is essentially $1.2 billion,” Rice said. “They’re obviously not worried about things deteriorating.”

Amazon said in a regulatory filing Friday that it already has made a nonrefundable deposit of $23 million on the buildings, and will deposit $28 million more Oct. 22, which it would forfeit if the sale doesn’t close this year.

In addition to 1.7 million square of office space leased by Amazon, the South Lake Union complex includes 100,000 square feet of retail space that Vulcan’s Healey said is about 75 percent leased.

Eric Pryne: 206-464-2231 or