Hmm What they could do with APTs is have 2 different ones - A large apt, and the current size. Price differenes of course between the two, but the primary difference is the large Apt (Lets call it a pent house) would have a slot space of 150, and a 'balcony the player could access from their apt that allows you 10 slots of garden stuff.

The physical dimensions of the apts would probably remain the same though. There should be big enough distinction to push for a house over an apt. But right now the incentive for apts just isnt enough yet.