"According to real estate sources, Zendesk is eyeing the market for about 125,000 square feet, just two years after doubling its headquarters on Market Street," wrote Temple. "Likewise, Practice Fusion is looking around for the same amount of space, Intuit is on the hunt for around 100,000 square feet and Microsoft is scouting for about 60,000 square feet."

Bloomberg's Levy had earlier reported that Google is also deep into serious expansion mode in the city: "Google agreed to expand its San Francisco office space at Morgan Stanley’s Hills Plaza building in what would be the city’s biggest lease transaction of 2013, according to a person with direct knowledge of the deal.

"The planned 10-year lease for 350,000 square feet (32,500 square meters) will increase Google’s space at the waterfront location by 25 percent, said the person, who asked not to be named because the process is private. The Mountain View, California-based company will pay average rent of $65 a square foot, little changed from the rate in its current agreement, which expires in 2015, the person said. "

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Yahoo to Move into Chronicle Building as Tech Office Grab in S.F. Intensifies