I'll start with, yeah it's kind of insane, but it just might work. I am by no means an engineer or home builder, so if there is something obviously undoable here, feel free to tell me. Have been kicking this around and wanted to see what it would take and then have an intelligent conversation with a builder at some point (or just forget about this all together.)
Our house is a walkout. Meaning the top floor is on level ground and then the bottom floor is still above ground in most places.
Upstairs we have a very large garage on the side of the house which has a concrete driveway and concrete floor. Underneath it is dirt/earth/whatever is naturally there that they used to probably grade that area when they built the home.
On the second floor there is a very large utility room (20x18ish) that was turned into a home gym. The entire wall on one side of the utility room is the dirt/earth/whatever that is underneath this area of the garage. I'm guessing there is an additional 5-6 feet of material going upward from where the ceiling of the utility room would end.
My crazy idea is to excavate out a small room going from the utility room, into the earth/dirt area under the garage. Have it finished with rebar reinforced stone/block, waterproofed as well as having a drain put in (easy because there is drainage in the utility room, would just require the floor to be cut and the pipes linked) and have a vault door put in.
My initial reservation was the structural influence an excavation like this would have, some feet under the garage slab. Would this be a concern if the room only took up 10 feet out of a 40-50 foot overall slab width? Would it need to be reinforced or is this just asking for a mine shaft to cave in?
The actual excavation wouldn't be rocket science but the other downside would be, it would be inside and using heavy equipment would be out because you wouldn't be able to run it indoors, let alone fit it though the door. I wouldn't be doing the work as I wouldn't even know what to do, so obviously I'd call a professional if this was even feasible.
So how insane is this? I can see it working, and I can also see this just being a disaster ;)