A unit can enter and garrison an unoccupied building if it can move into base contact with the building during the Movement phase. Unless differently specified, only infantry, war beasts and swarms can garrison a building. War machines can be deployed in buildings, but only if there is space to deploy the model on the top floor. Units cannot garrison a building in the same turn in which they have marched or made a reform.

It is perfectly acceptable to place a representative model from the unit in, on or next to the building to remind you that the unit is inside (standard bearers are ideal), and place the rest of the unit aside from the battlefield.