it's the top floor - does anyone know which floors John and Angus lived on?
Lots of pictures of the inside which is pretty cool - although to say it's been gentrified since 1965 would be an understatement.
There's also a shot of the rooftop which, judging by the large building in the background, would seem to be the same rooftop where this Donald Greenhaus photo was taken:
"The loft for sale spans the whole fourth floor. The current owners are the artists Giancarlo Biagi, Jill Burkee Biagi, Germana Pucci and Paul Lucchesi. The Biagis and Pucci own the fifth floor where Cale once lived."
This page about the Velvet Underground on the Pop Spots NYC site https://popspotsnyc.com/lou_reed_velvet_underground/ also has info about 56 Ludlow Street. It also correctly identifies the roof where the photo of the band was taken, but it's not on top of 56 Ludlow Street as stated in the article, but on top of 54 Ludlow Street (the house to the right of No. 56).
Having analyzed the rooftop photos I must say it's difficult tell if the band was photographed on top of No. 56 or on top of No. 54. The Pop Spots NYC site correctly identifies No. 56 as the house where Cale lived, but then shows the roof of No. 54 (the house to the right of No. 56) as the location of the rooftop band photo. The houses in the background of the real estate photo of the roof of No. 56 on the other hand seem to suggest that the photo of the band was taken there.