Prevent companies from owning housing without having the increased taxation. Treat married couples as a single unit (this may be problematic with separated couples?). I don't have a good answer for children except that then you are effectively gifting all your kids and relatives a house. So if THEY want to buy their own, different house they pay increased taxes or sell the previous. And that prevents one person from owning dozens of properties; divesting them to their children. Which really means if your goal is to property horde you need more kids...which is a very weird incentive to be sure.