Actually, I think that we should build more single family homes or detached townhouses (there are such units) and abandon the mantra about preserving farmland that never gets farmed or limiting urban sprawl.  I don't know how many condos are empty, so you might be correct there, but I do know that ground level detached homes are what seems to be desired by families especially.  I can also tell you that ground level houses don't stay empty very long before thieves break in to steal copper pipes or whatever they can find, but condos on the umpteen floor could stay empty for decades and nobody would know.