The city’s hottest project has also been its quietest — up until now.  Zeckendorf Development and Atlas Capital Group’s 80 Clarkson reported its first 20 contract signings in an amendment filed with the New York State Attorney General’s office. The total is just enough to push the development over the 15 percent threshold to declare its offering plan effective. The deals include a contract for a full-floor unit in the building’s west tower, asking $75 million. The 30th-floor unit spans 7,300 square feet, making it the largest single residence in the development, and sought more than $10,000 per square foot. […]

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