As far as a dirt floor goes, the spillage of powder could build up and eventually become a fire hazard. If you can, put a concrete floor in that area or a plywood floor so that cleaning spillage would be easy.
No dirt floor in either potential area. My shop has a cement floor but is off a garage area with a dirt floor.
I think I'm going to set up in the outside shop and just CLP the crap out of the dies and press unless someone tells me the storage room in the house will be okay.
My reloading setup is in the garage. I started out with a Lee challenger kit (gift for Xmas), then got a Lee Classic Turret, and now have a Hornady LNL AP for pistol/5.56 and a COAX for precision rifle.
I really like the Hornady One Shot Cleaner and Lube for dies and press stuff. It leaves a dry protective film that seems to keep corrosion at bay. I spray my dies and powder drop/linkage liberally and they seems to be holding up just fine in my garage (not climate controlled at all).
Then again, I live in the desert. BUT my 2.5 acres is a lake right now due to 4 ft of snow melt-off in this 50 degree weather. It's not uncommon for us to have 40 degree swings in temperature day to night/week to week. I pull my truck into the garage wet every night (because I drive through the lake to get home) so the humidity is above normal. I haven't seen any signs of corrosion since I started using the Hornady One shot lube.
I store my primers under the bench in a heavy duty cardboard box and my powder up on the top shelf above bench in the original containers (or a smaller 1 lb jug I transfer into when using). I will not comment on whether my quantities fall withing the NFPA guidelines. I will say that my chemical storage (gas cans, acetone, denatured alcohol, aerosol lubricants, random flammable stuff, etc...) is more of a concern than the powder/primers. I deliberately put all that in the farthest corner of my garage as a precautionary measure. Heaven forbid but if my garage ever burns, it could be rather intense burn (albeit a brief time period).
Long story short, you should be fine in the garage.
Cheers,
Toby