Posted By: 
Jason DuncanBrian-
I am a builder in MO.  Since we tend to have alot of tornadoes I build all concrete "safe-rooms" in my basement.  If you have not built yet, I would suggest that you run this by your builder.  If you are no pouring a basement foundation, then metal studs on a slab would have to.  Another alternative is a stacking form system where you can pour all your exterior walls in concrete.  If you go that route, just have the builder form and pour a small room in the house.  You will have to alternate your house plans of course.  This all would cost a little more money, but you would have peace of mind in the end.  The cheapest space you will get in your home as far as adding square feet would be a poured basement, and in this case pouring a safe room to the earea would be easier and would not alter your main level floor plans.
Regards,
Jason