I haven't built a dedicated fish room yet, but I've built the garage on my house and finished our walk out basement to a master bedroom suite. The bathroom in the basement is below grade. While I'm happy I went with extra insulation for the walls (I've learned it's hard to over-insulate), I wish I'd have sprung for the under-tile heating elements. We don't have the worst winters in MO but the chill off the floor in winter is brutal. If you go with something other than bare concrete floor, consider something like that to help keep your temp stable?
The garage is more like a swiss army knife that poses as a garage. It's man cave, craft room, office, storage, greenhouse for the garden in early Spring, and now houses extra tanks. Biggest lesson learned there that seems highly transferrable to the aquarist hobby is over planning the electric is about as tough as over-insulating. Oh, and outlets belong on ceilings as much as they do on walls! Surface mounted boxes with flex conduit runs make for easier reconfiguring down the line if things evolve unexpectedly.
From conversations with locals, reading forums, and YouTube, venting/moisture control seem to be a common topic for basement builds. Overshooting here, too, may be worth considering depending on your situation.