There is plenty of developable land already available without touching the greenbelt. Landowners just aren't building on it. They are holding as it goes up, up up without any effort. Greenfields development has an endpoint--that is unbelievable and destructive traffic congestion. 
Think about redeveloping Scarborough for example--lots of land that could be just slightly denser with much nicer homes. Lots of parking lots without cars. Too many nondescript tiny 900 sq ft homes on large lots. But Toronto's planning department calls these 'stable neighbourhoods' that are to be protected--never to change. Look inside the Toronto planning bureaucracy and the political councillors for the answer to part of the problem.