Won’t this violate fire codes?

No. You are not a “permanent residency” and overnight is not “common” or “your primary purpose”. Kingsport’s Fire Marshall has the weekly Hosting Schedule so they know if people may be in the building if there is an emergency. The Fire Marshall already inspects your building as a public gathering place. They understand FPGK and impose no extra burden on the church. We do advise you to check your smoke detectors and fire extinguishers regularly as any prudent facility manager would. A quarterly inspection as you prepare to Host would be a good time.