So involve a structural engineer, and architect and the building inspection unit of your local government. Figure twice as much to plan it as you have mentally budgeted to build it.
I added on a solid brick extra large two car garage but it was alongside the house and I has to put a 8" (IIRC) lip around the half stairs to the basement to prevent low flowing gasses from going from the garage to the basement. When I took the building plans to the local plans approval agency, they took a look and asked me if I was going to park a tank on the slab, it was so freaking over-engineered.
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