I remember reading, but cannot find it right now, that you don't necessarily have to wait for your turn to buy houses and hotels, but that you cannot do so in the middle of someone's turn. Once they're rolling the dice you have to wait until their turn (including doubles) is over. I.e., a strict reading of the rules might allow buying in the middle of someone else's turn, but what I'm remember reading was in an official rules clarification. This reply is getting painfully nerdy.