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In my continuing search for the most ideal way to construct a secure home or remote retreat with elements of survivability, stealth, off-grid living, and yet keep it within the bounds of conventional financing, I keep coming back to a version of the same idea; to build under your garage, especially if it is attached. In the Northern region where I am (Michigan), since one must dig down four feet for foundations to safely be below the frost line anyway, the additional cost of going another 4 or 5 feet isn’t very much. In fact, with the home I just completed, I figure my additional cost of building a retreat under my three car garage was about $25,000. This includes the Fort Knox vault door, plumbing, electric, HVAC run and return, treated floor, and hydronic heat I put down myself. With 780 square feet of living space, that’s about $32 per square foot for space that would have otherwise been filled in with dirt. Try to add on to your house for that cost. Besides the costs, though, it was something I could do even with subdivision building restrictions which would not have allowed me to build a bunker
The concept is simple and made easy by using reinforced poured concrete walls 10” thick, and by using Span-Crete® or any pre-stressed concrete product, which in my case covers a 30 foot open span ($10,500). Often, for not much more money, you can have them add more steel to take even more weight. I know someone who did this so he could drive in his 13,000 pound Bobcat into his garage with bunker under it. The two foot wide segments are quickly set in place by crane ($500), then they are covered with 2” of hard foam insulation, a 60 mil over-sized rubber roof membrane ($700), and then 4 inches of concrete is poured over that (which you were going to pour for your garage floor anyway, thus zero additional cost). I curled up the edges of the rubber membrane against the concrete walls of the garage (which come up about a foot then are wood). After the garage floor was done, I cut the excess member off about four inches up, then covered it with 2×6” treated wood, nailed to the wall and caulked it. The rest of the construction is conventional. There is a main doorway accessible from the basement through what looks like a closet. It goes down a few steps because it is slightly lower than the rest of the basement, and has a separate sump pit and pump out. There is a Fort Knox inward-opening vault door so that if the house collapses, the door will not be blocked by debris since it opens into the retreat space. I also recommend a mechanical lock, since electronic locks could be destroyed by EMP (how frustrating would that be). Some people talk about blast doors. IMHO, if a vault door is not enough, you had better move further away from ground zero. Six inch diameter PVC was used in various places before pouring the basement walls for HVAC forced air in and out, also with two separate air vents, intake and outtake, and two more to run electric service and hydronic heat hook ups through. Though my lot and situation did not allow it, a secondary entrance/exit is a very good idea. Mine is unfortunately a pick axe. To save on another vault door, you can use an old gun safe and torch open the back as a walk through. Spend the money to have a good contractor seal and insulate the exterior walls, such as one that offers a dry basement guarantee of at least 10 years ($800 more for me–the entire house was $2,600). For the basement floor, I used Rust-Oleum basement floor sealer. I also used the non-skid additive, and it produced a very nice finish ($150). Just be sure to ventilate when you do that or you will have a headache. Electrical is simple, just conduit to outlets all around on the painted concrete walls and ceiling, and regular ceramic light fixtures with efficiency bulbs. A great place to have put the generator would have been under the stoop of the front door, had I been a better planner. Mine is out in the open, but I am putting in a DC backup system that also runs to a solar panel on the roof. So what you finally get by doing this is the addition of highly secure space to an otherwise conventional home that most people would never expect to be there in a residential home, under where you park your vehicle. Since this was less than 20% of the cost of the house, and added a lot of “storage space” or could be a “home theater” room, the bank didn’t have a problem with it. On the plans, it just looked like more finished basement space. One more thing, I also ran plumbing into mine to allow for bathing. (A shower, not a tub). One of the first things that I’ve noticed about the bomb shelters and safe rooms that I have seen is the lack of a toilet. Even if you don’t want to do the expense of running plumbing, be aware there are many vented dry toilet or marine type (pump-out) alternatives. If you are going to spend that much, I say at least spend a little more an make it civilized.- Rourke

JWR Adds: For new construction, I recommend going to the expense of putting 10″ to 12″ of reinforced concrete overhead. That is sufficient to make your basement double as a fallout shelter. But that upgrade will of course make it obvious to the building contractors what you have intended. A ceiling of say eight inches thick probably wouldn’t arouse suspicion. Perhaps a “do it yourself” second pouring of concrete would work (IF the floor beneath is engineered to take that sort of dead load), for those of you that are Secret Squirrels.

I also recommend that you fully conceal the entrance to your shelter. There are a number of ways to make a doorways disappear. Anyone that is relatively skillful with hand tools can build a pivoting bookcase door. (Tres Batman, Tres Chic.) To make the doorway less apparent, first remove all of the molding and then lower the top of the doorway from the standard 78 inches to perhaps 60 inches–filling in with framed rectangle and sheet rock. (Of course you’ll have to be familiar with how to frame with 2x4s, cut sheet rock, tape, and texture to make this look right.) Then you can position a five foot tall bookcase in front of the the doorway. Yes, you will have to stoop each time that you pass through, but the entrance will be far less perceptible to all but the most keen observers. BTW, there are lots of similar ideas in the slim little tome titled: “The Construction of Secret Hiding Places” by Charles Robinson, (1981) published by Desert Publications.

Two inexpensive approaches to basement shelters that I’ve recommended to consulting clients are: A.) Making a full size basement appear to to be a “half basement” by the addition of a solid wall or false wall. (Either make a hidden door through the false wall, or a trap door to the walled-off room from a room upstairs) and B.) Making a basement disappear completely, by concealing its entrance (as described above) and by using some earth berming to hide any exterior evidence that the house ever had a basement.