The following sign just appeared...
So it looks like a new building with additional floors... and by fall of 2012? Seems awfully ambitious... Looks as if the new owner bought the building in 2008. According to the Corcoran listing:
Four story building built in 1901 with an additional two stories of air rights. Currently configured as a three family with an owners duplex, and two floor through two bedroom apartments. Large south facing garden. Excellent opportunity to create a architectural dream home.
Streeteasy has the closing price at $3.2 million. We're tracking down more details on all this now.
Meanwhile, word is that this was the onetime studio of sculpturist Louise Nevelson. A look at Property Shark notes that she did own the building at one point.