One Young (née Mayfair Hotel) | 5 fl | Complete

[panamaniac]

That is correct (the two buildings share a common wall, I believe).  Some heritage elements of the six storey tower are to be saved as well. 

What heritage elements does the "tower" extension have?  They are not immediately obvious to me, just from looking at the photos.

(I don't think there can have been much of an approval process back when Mayfair got the permit to add those storeys ...)

I'm not able to pull up the original notice of intention to designate, but if I recall correctly, it has to do with the art deco capitals of the pilasters, the entablature and the details of the windows.  Some examples of those elements are supposed to be salvaged as part of the demolition, iirc.