An assault isn't resolved in the same manner as a regular combat, as the two units involved are not in base contact with one another. Rather, we assume that a swirling melee takes place around the building's door, windows and other entryways, with the best fighters from either side pushing their way forwards into the fray.

It's hard to maintain the impetus of a charge in a confined area, so models assaulting or defending a building lose all charging bonuses. This means, for instance, that they do not receive the normal +1 combat result or Initiative, gain no charge bonuses from weapons (like spears, lances and pikes), and do not make Impact Hits.