Easy answer is yes you can.  the much more difficult thing is actually doing it.

You would need to dig out that area, while reinforcing a footer and walls under the slab to hold up the garage floor.  Also you would need to know if garage floor is thick enough to support the weight without dirt under it.

Funny thing, My fraternity house dug out a Basement under the house to use as a party room.  

I spent many days laying block to fill in the space between the new footer and the exisiting floor joist.  

We cut 8-10 linear feet at a time around the perimeter and then built new footers and walls.  

After the perimeter was done, we cut the dirt from the center of the room and poured a floor.  And we had a basement.   Took several years to complete.

Now that I am older and wiser hopefully, I would consult an engineer and get their opinion.

how is the garage floor (slab) attached to the foundation, Is it supported by the foundation?

You might be able to dig out, reinforce, then park car.––-maybe

If its not, then you won't be able to dig out/reinforce w/o a lot of asspain

You also need to know the composition of the slap, to see how much reinforcement you will need.

The easiest way to do it, cut the slab out in the garage, dig out, add supports then pour a solid slab/reinforcement.

If you want to park a car on top of it, it will cost.

Also get a engineer to spec the support/slab

I doubt you can do it without damaging the garage floor or the garage, they will need to excevate down for foundation walls and possibly a footing in the center for a support collum in the center.  Our garage is supported with a 4" steel post in the center and a 12" I beam running down the centerline of the garage.  But it was built with the house.

As stated above, consult an engineer. Why not just wall in the small section of the basement and use a hidden door?

I was a builder in another life, but won't claim to know everything.  Is it possible, yes.  I've retrofitted areas under porches with poured concrete.  There are several issues you'll need to address.  The garage floor and the doorway.  We'll talk the doorway first.  If you plan to cut the existing basement foundation wall to make a doorway.  Its doable, and not terrible hard, you can hire a concrete sawing company to cut the walll.  They'll need to make two horizontal cuts for the header to give claerance to drap the slab.  But be forewarned, the slab for a 36-40" doorway, is going to leave you with a large slab of 8" thick concrete laying in your basement.  It's going to weigh well over a ton.  And when it drops you'll be luck to not break the existing basement floor.  Its reforced with rebar, so breaking it up is tough.  You'll need most likely have to have it cut up to haul it out.  And they charge for cuts by the inch/foot.

The existing slab is free floating and not tied into the foundation.   The garage floor will need to be repoured with renforcements under tension to compress the slab. This should allow you the strength needed to park a vehicle on top of it.  This is typically done on new construction with preformed poured panels, then coverred with a secondary slab.    And you'll need to build supporting foundations under the permiter (maybe in the center depending on the size) to support the slab.  

Probably the easiest means of doing it would be to bust out the existing floor in the garage.  Then you can use power equipment to dig out the room.  Then you can build/pour supporting foundations.  You build and support a joist and plywood floor to pour the new slab on top of.  Drill into the foundation to tie the rebar from the slab to the foundation walls.  You'll need to consult local engineering to ensure what you have poured meets guideline/code.  If you go this route then you can cut the doorwar like above, but dig a coffin type hole inside the room area and drop the slab into the hole.  Then bury it when you pour the new floor.  Don't forget to address drainage tiles.

Its not "hard", but its a lot of work.  And its not neceassily cheap.  If your just looking for more space and OPSEC, could be a good route.  If you just want security to store a few things from fire, you'd be better off building a poured block wall roomin the basement and fill the bolck with concrete.  Same as what you would have under the garage, minus all the cutting, digging and pouring....

We worked on a 6K sq ft. house that had a seperate garage with a water tight basement.
Floor was 4" x 12" rough sawn (Cypress I think) with a steel center girder.

The home owner wanted a water storage area to feed his Dodge forest service truck complete with fire nozzle, they had lost a house to fire years ago.

I was amazed at the workmanship and engineering.

It would probably be easier to just build a wing off of one of the basement walls so you don't have to worry about the car crashing through the floor or going to the trouble of a new reinforced and supported garage and garage floor.

Just cut a whole, put  header in somehow and burrow your escape tunnel

If you don't need a 6'5" tall doorway, it would probably be pretty easy to do.  Think a arched portal.  THen you wouldn have to waterproof your new place and/or put a water proof door in the hole.

You could have them add a porch onto the front of the house and put the secret room under the porch.

Man, I was just thinking about the same idea.  The only difference is, my garage floor has sunk and cracked, needing replacement.  
What if you don't do the whole area?  Does anyone know how far I could go unsupported if  the floor was poured 6" think and tied to the foundation walls around the edges?
If the room was 1/2 the width of one side of the garage and ran the length of the garage, it would use 3 existed walls and the new 4th wall would run down the middle of one side, thus supporting the garage floor.  Would that be enough support?  I'm guessing the free span would be about 6' wide.

sweet!  Did under the neigbor's house

Garage slabs are generally poured inside of the wall foundations and are solely supported by the compacted dirt below the slab.  Undermining the compacted dirt would be very difficult by hand and will result in settling over time and will crack your slab.  

It would be much easier to just rent a backhoe for a day and dig in your front yard right next to your garage.  Knock out a doorway to the basement and block up a new room with a well supported poured concrete roof covered with a waterproof membrane.  Cover the roof and membrane with 6 inches of dirt and reseed your grass on top.

I'm guessing in wanting to do the work himself, Forest also wants a certain level of opsec that a back hoe in the yard  fails to provide.





But I agree that the slab of a garage floor is likely strong enough to support tunneling.   Now, I've seen crawl space houses tunneled out for basements.... and that worked, however the house wasn't supported by the dirt that was tunneled out.  





 

Like my Stepdad (builder, master plumber, master electrician, HVAC etc) always tells me. "You can do anything if you are willing to pay for it."

J-

Anyone else think of the thread about the guy who wanted to build a second story on his garage and the HOA told him no?  Guy just dug up the garage and build a basement under it.  Very nice work.

www.msprecast.com or flexicore

No.

Already been done, garagejournal forum.  I'll search for the link.

http://www.garagejournal.com/forum/showthread.php?t=12234

Wowee wow wow !!!

His neighbors must have LOVED that.

Thats what I was talking about!

Holy Shite!! They dug it right up to the edge of the neighbors driveway?!

My wife's friend's husband is a foreman for a concrete contractor.  He said garage floors are not steel reinforced (unless specified by homeowner) because they do not experience extreme bending moments or shear stresses because well compacted earth supports any weight from vehicles directly.  They also do not experience frost heaves and while they get a little colder and warmer in winter and summer, they don't experience extremes.  He also said garage floors are only about 3" to 4" thick.

Shame he took down the pics. Wonder how it turned out. ( Can't blame him I guess if people were asking for his address etc. )





 

not a good idea.

One minute you will be sitting in your "bunker" under your garage, the next you will have the grocery getter and a load of concrete sitting on you.

I suppose you could shore it up somehow, but man it will be a total mess and take a lot of support. There is probably no reinforcing steel in your garage slab either and it will fail in a hurry trying to support someting over a span.

Lots of bad info in this thread.  I am a contractor.  Your current garage floor is non structural.  To build a room under it the current floor will have to come out.  Then you excavate the garage out.  You most likely will have to underpin the garage foundation.  Chances are the garage foundation does not go down to the basement footings.  This will be a huge cost.  Then you need to install steel beams and steel decking.  Then you pour a new floor.


Basically, this will be a very high cost per a sq foot.