It is not a written rule yet. its only a law that you can't have illegal item on you, which means that you are allowed by the server rules but the police might take it if they see it. so I belive that you can buy a house in any zone and store illegal items along as you dont use it to store drugs and sell hem from the house as you are not allowed to do that with a vehicle. however the police might raid your house for illegal items if they have reasons to belive you are storing illegal items there.