From the Monopoly rulebook:
If there are a limited number of houses and hotels available and two or more players wish to buy more than the Bank has, the houses or hotels must be sold at auction to the highest bidder.
I don't quite understand this scenario. I thought that in Monopoly you could only build on your property on your own turn. Given that, I don't see how the scenario of two or more players trying to build at the same time could happen. Is my understanding of the rules incorrect? Is building houses on another player's turn allowed? If so, can it be done after the player rolls the dice and lands on your property?