I built my reloading room in a corner of my 30x36 pole barn. It measures 6x12, which sounds awfully small, but it easily accommodates a Dillon XL-650, an RCBS Rockchucker, lots of shelves and a cabinet, and all of the accessories I need. It's well insulated and I built an air conditioner into the wall and have an electric space heater inside. I am very satisfied with this setup.I live in a small cabin in the woods of Michigan's Upper Peninsula, where the weather and humidity vary enormously.
I keep my powder, primers, ammo and other stuff in another outbuilding on the property. Most of it is stored in airtight containers. Despite the wide range of conditions, I am still using WWII surplus H-4831 and H-4895. The stuff is still fresh and crisp, and I have never had a primer go bad, even though some of my stash is 30 years old. I think a good container is more important than climate control. After all, the powder and primers in military ammo stored in the desert seems to last an awfully long time.
I don't have a basement or a large home, so my options were limited. This setup works wonderfully for me.