It's pretty cool looking and huge:
http://www.tate.org.uk/about/projects/tate-modern-project

This new development will transform Tate Modern. An iconic new building will be added at the south of the existing gallery. It will create more spaces for displaying the collection, performance and installation art and learning, all allowing visitors to engage more deeply with art, as well as creating more social spaces for visitors to unwind and relax in the gallery.

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Right now, the main bulk of it looks to be finished but there are no windows in or anything. Seems a bit odd. The Turbine Room isn't being used any more it seems so why not build in that space. Or even build in the chimney stack?

Couldn't they have spent the money on that building a brand new Tate in Birmingham, to try to de-centralise stuff a bit?