I reckon this should be categorised as more of a dream than a plan. There are so many potential pitfalls – not least the Owners Corp refusing to sell you the space, which as other posters point out, is theirs to do with as they please. The thing to remember is that just because the attic is above your apartment, it doesn’t mean you have any greater claim on it than any other person in the building.
A much better plan would be to buy into a building that offers the potential for upgrading by buying a bigger unit when one becomes available (and maybe renovating that). But, as ScotlandX points out, there’s no free lunch in strata – any ‘profit’ you make by extending into common property goes straight to the Owners Corp.