As models are not physically in base contact when fighting in a building, a little abstraction is required when working out combat.
It is hard to maintain the impetus of a charge in a confined area, and so models charging a unit inside a building lose all charging bonuses (including striking first, Strength bonuses for weapons, and so on).
For both the attacking and the defending unit, a number of models up to a unit strength of 10 may fight. For example, if a unit of twenty Goblins was assaulting or defending a building, ten models could fight. If the unit consisted of eight Ogres, up to three models could fight, as Ogres have a unit strength of 3 per model.
If characters, champions, or other models that can normally be singled out if they are in base contact, are involved in the fight, up to a unit strength of 5 of the enemy models fighting may choose to attack them (as often such heroes will be in the thick of fighting at doorways or on stairwells).
A single challenge may be issued and accepted as normal (within the unit strength 10 of models fighting and not in addition to this).
For special attacks and items that affect models in base contact, a model fighting in a building is assumed to be in base contact with one enemy model nominated by the controlling player - unless fighting in a challenge, of course, in which case they are in contact with the model they are fighting!