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 ;)

I am sure you will have to cut the floor out of the garage and poor it with steel panels and footing and cement walls, all that would leave you with a very low ceiling. I am pretty sure code will not allow it.

What if I made a small hallway with steps that went down from the "entrance" coming from the utility room to give me whatever depth I'd want?

I'm guessing there's nothing under the garage and with going a few feet out from the utility wall foundation, there would be nothing there except dirt and rock.

Earth excavations can be dangerous.

A good contractor should be able to give you a thumbs up/down in 20 mins and will more then likely cost you nothing

YA, someone really has to look at it, a step down would work as long as it drains.

Had planned on it, I just wanted to rule out 'are you fucking insane?' first.

it is highly doubtful the slab was built for any kind of unsupported span. However it is certainly feasible to cut out the ceiling section and replace with a properly engineered ceiling.

It would be a lot easier to dedicate a corner of the garage, say 10x10 and build  a concrete block room.

It's kinda like the old race car adage: Speed costs money, how fast do you want to go?
You can do anything you can afford.

Code does not have a lot to say about a small room that is not 'living space.'


I have installed vault doors through block basement walls that we dug down from outside.


You can get very close to a foundation wall from the outside with a good backhoe operator.

My guy could leave about 6 inches and it would often just fall into the hole and leave the foundation exposed.

We made them short so we could put insulation at the top and still have some cover.

The AHJ did not even require a permit.

There are companies who will cut a hole in your garage floor, excavate it, then install a safe type room. Don't know what it would cost though.

If the garage is large enough cut the slab and drop in a septic tank then restore the slab with an opening.