The only way to take a building after it has been occupied by an enemy unit is by the bloody tactic of storming in with one of your infantry units. This is called an assault.
Each occupied building may be assaulted by a single infantry unit during each movement phase. The assaulting unit must declare a charge against the unit in the building. There can be no multiple assault charges against a building, as the assaulting unit is imagined to surround the building and try to break their way through windows, doors and other openings. If multiple units that must charge enemies in range (such as Frenzied units) are in charge reach of an enemy-occupied building, only one must charge and the controlling player may decide which one will do so.
Assault charges are resolved just as if the building were the target unit. If the unit's charge move allows it to get into contact with the building, it is successful, otherwise the charge is failed as normal.
A unit in a building may hold, flee or stand & shoot as normal, and is subject to the rules for shooting and fleeing from buildings (given later). If the unit flees the building in response to the charge and the assaulting unit has enough movement to reach the building, they will immediately occupy it.
Note that a charge during the movement or magic phase is the only way to initiate an assault. Other situations that would normally trigger a charge (such as a pursuit into fresh enemies during the combat phase) cannot initiate an assault and units must stop 1" away from the building.
An infantry unit inside a building may not declare charges, even if it would normally be forced to do so because of Frenzy or some other similar rule.
If an assaulting unit is engaged in combat with any other enemy unit (by a counter-charge or some other special rule or odd special circumstance), it will not fight the assault, but it will fight the enemies outside the building instead. The unit in the building takes no part in the fight whatsoever and, if the assaulting unit is still in contact with the building after the normal fight, the assault automatically fails (as described below).