A Chaos Siege Giant may always choose to attack and destroy buildings, regardless of the scenario (see the Warhammer rulebook and use the Watchtower description for any fortified buildings where appropriate), and may always choose to assault the building even if it is occupied, potentially brining it down on any unlucky garrisoning troops inside. A Chaos Siege Giant always attacks a building with the Smash with Pick attack and need not roll for any random attack type.