OK, im sorry, I've been misunderstood.
As of right now there is no current
pratical way that you can stack with triggers.
If you wanted the buildings stacked in the first place you can just do it in the editor. Save your space on triggers.

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Basically they can't stack by building stuff, like a human could. Only way would maybe have it detect if the computer built something and combine it with what I said before and create the building on top of something else.
But yea nothing practical.