Amazon.com the world's largest online retailer, agreed to buy its 11-building headquarters complex in Seattle for US$1.16 billion in a deal poised to be the biggest US office transaction of the year. The campus has 1.8 million sq ft of space in the South Lake Union neighbourhood, Amazon said in a filing with the US Securities and Exchange Commission. The company said it's made a non-refundable deposit of about US$23 million, which will increase to US$51 million on October 22. It expects to complete the purchase in the fourth quarter. The terms of the agreement would make the transaction the biggest purchase of 2012 for a single office property anywhere in the country, said Dan Fasulo, managing director at real estate research firm Real Capital Analytics. Seattle office buildings have been among the most desirable properties for investors this year, the firm's data show. Vulcan, billionaire Paul Allen's investment company, put the campus, which is now leased by Amazon, up for sale in August, saying it would use proceeds from the sale for other real estate projects in the area. South Lake Union is one of six areas Seattle designated as "urban centres" in the city's 2004 growth plan. "When we started the disposition process, we did not think Amazon would necessarily be the buyer," said Ada Healey, vice-president for real estate at Seattle-based Vulcan. The 11 buildings were built in six phases. "We were prepared to have the six phases go to six different buyers." Healey declined to comment on the deal's capitalisation rate, a measure of yield used by real estate investors, or the potential profit for Vulcan. CBRE Group, the Los Angeles-based listing broker for the Amazon campus, declined to comment on the filing, said Robert McGrath, a New York-based spokesman for the real estate services firm. Drew Herdener, an Amazon spokesman, did not immediately respond to telephone calls seeking comment. The Amazon campus is the largest of more than US$3 billion of office deals under contract or on the market in Seattle, Kevin Shannon, vice-chairman at CBRE, said before the agreement was announced. "I'd estimate there will be more than US$4.5 billion closed by year-end - more than triple last year's total," he said. "Capital follows markets with the best fundamentals. Seattle has some of the best office market fundamentals in the nation." The announcement was made after the close of regular US trading. Amazon fell 0.8 per cent to US$258.51 yesterday. Its shares have climbed 49 per cent this year as updated Kindle e-readers and tablets proved more popular with consumers.