You cannot simply buy a house because you have the cash; you must first satisfy these conditions:
: 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. when can i buy a house in monopoly
: You must own all properties within a specific color group (a "monopoly") before you can build any houses on them. You cannot simply buy a house because you
: None of the properties in that color group can be mortgaged. when can i buy a house in monopoly
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.