AGREED! And check this site out for some inspiration. Some seriously bad ass stuff:
http://hiddenpassageway.com/
I think I'm with everyone else with the outdoor structure overall, though I understand the lot restraint issue. It just seems to me that a bunker would be tough to really get much of anything back on when it comes time to sell the house where as an outdoor structure could easily be converted from a brew house to a guest house.
There isn't enough lot space to add an external structure. Besides, the house itself looks pretty big as it is. I like the idea of it being hidden underground.