A unit that wishes to attack an enemy-held building must adopt a skirmish formation in order to move through doors or windows. This is automatically accomplished as soon as the charge is declared. Attackers can move straight through any unguarded doors or ground floor windows but must stop if there is a defender guarding the entrance.
Up to two models per side can fight across a window or doorway. The defenders benefit from the rule for fighting across defended obstacles, so the attackers will need 6s to hit regardless of their Weapon Skill characteristic (WS) until they win a round of combat.
When both sides have troops inside a building, and assuming that models cannot physically be placed inside the building model itself, it will be necessary to resort to a map or ground plan. Quickly draw out a rough ground plan of the building noting the position of windows and doors. Place the ground plan as near to the gaming table as possible. The defender places his models onto the ground plan and attackers are transferred over as they move into the building. Further movement and combat can then be resolved on the plan, and models transferred back to the gaming table as they leave the building.