Is that considered a houseplant, too?

I don't think it is a house plant, it would grow inside large greenhouse.

Would it be able to be overwintered inside an unheated garage, and go dormant? So far I've noticed the garage hasn't gone below 50F, of course the coldest it's been since I've moved to this house has been 30F and it's only early November. It also has a nice west facing window.

Morgan, whereabouts in the UK are you? Are you sure you can't overwinter it outside, against a house wall for protection, with bubble-wrap around the pot, and fleece around the plant when temperatures drop really low?

I live in central England and I have wrapped freece around my tree fern. As the first frost's have arrived.

they are tougher than you might think it really should be ok with a fleece wrap and as its under a carport i assume it is covered so i should be fine unwrapped mine have taken -9C with no problem if it is covered be careful not to let it dry out over winter as it will hinder its start next spring or even kill it if it gets really dry
good luck and happy growing
oh BTW tree ferns are addictive be careful i now have about 9 species of cyathea 3 species of dicksonia and a couple of other trunking but not strictly speaking treeferns