: Some interpretations suggest you can buy houses even while the dice are in the air, but many players agree that once the dice land, the rent is fixed based on what was on the board at that exact moment.
: You can technically build while another player is moving, though common "gentleman's agreements" suggest waiting until a dice roll is completed.
While the "at any time" rule is broad, there are practical limits and community etiquette to consider:
: You can build between the turns of other players.
The official Monopoly rules state that you can buy and erect houses " at any time ". This flexibility is often misunderstood, but as long as you meet specific property requirements, you don't even have to wait for your own turn to start building. Core Requirements for Buying Houses
: You must build houses one by one across the entire group. For example, you cannot have three houses on one property and only one on another in the same group; they must stay within one house of each other at all times.