In the crush of a building, brute force or skill at arms counts more than any other factor. As such, only wounds inflicted by each side are counted for combat resolution, no other points are scored (note that musicians still have their normal effect in case of a draw on combat resolution scores).
If the result is a complete draw or the assaulting unit loses the fight, the assaulting unit has failed to storm the building and the combat ends immediately. No Break test is taken, the assaulting unit is simply moved back 1" (still facing the building), and is not engaged in combat anymore.
If the defending unit loses the fight, it must take a Break test as normal. If the test is passed, the assault
is failed and the attackers moved back, as above.
If the defenders break and flee, or are wiped out, the assault has succeeded. The assaulting unit cannot pursue the fleeing enemies, but it occupies the building instead, if it fits inside (this counts as pursuing for the purpose of capturing banners)