Those ac ledges are about 6 sqm. Agree that it is a waste of space.
It is a pity that URA left this out when it tightened the rules of bay window and planter boxes few years ago.
http://www.ura.gov.sg/uol/circulars/...l/dc08-17.aspx
I also notice that some commercial building in AMK Industrial area also have such wasted ac ledge design.