A Chaos Siege Giant may always choose to attack and destroy buildings, regardless of the scenario, and may always choose to assault the building even if it is occupied, potentially bringing it down on top of any unlucky garrisoning troops inside.
Source: Warhammer Armies Project: Unofficial 9th Edition (v2.2)
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A Chaos Siege Giant always attacks a building with the Smash with Pick attack and need not roll for a random attack type.