Interesting. The link from You Need a Thneed above indicates a 748,000 square foot mixed use tower. My understanding of the current zoning for downtown is that CM-2 only allows for a maximum floor area ratio of 20. (Based on page 187 of this document
http://www.calgary.ca/PDA/DBA/Docume..._of_1p2007.pdf) Since the new downtown bylaw hasn't passed yet, this should still apply. That would imply the piece of land their building it on needs to be 37,400 square feet. Since the existing building is 3 storeys with 27,000 square feet of leasable area, I very much doubt the lot is that big.
I can think of a few possibilities:
1) I'm misunderstanding the bylaw, and more than 20 FAR is allowed
2) They're using the adjacent land to Art's Central, the building to the west.
3) Council made an exception, and is letting a higher FAR go through.
Any ideas on which it is?