Better yet, a picture example and the wiki page for it:
http://www.atitd.org/wiki/tale7/CompoundIn the case of ATITD, compounds can be expanded at will. There's an initial minimum build size, and it can be expanded one or more tiles after it is built. There is a maximum size it can be, though, of 50 tiles.
For other games I've seen, you can make different sized buildings, but once built, you can't directly expand the building, though there may be add-on structures and such you can build to expand with. It's been too long since I've played a couple of these, so don't recall the details properly.
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