We considered building a sun room for our hot tub before we got it. We also consisdered putting a roof & screening around the deck and setting it into the deck (as in on the ground).
We finally decided to put the sucker outside on the lawn for a year to see how we liked it, since everyone was always telling us it should be outside. We live in Canada, so it's cold here in winter too.
We have discovered some interesting things.
1. the only weather we will not go tubbing in is thunderstorms. Everything else is fine. The worse the weather, the better we like it. A Tilley hat keeps rain & snow out of my face and ear muffs take care of the cold if it's cold & windy. Just cold is fine.
2. watching the planes go by and seeing the stars is great. Sitting in the tub while it's snowing is sooooo cool.
3. if we did have a t.v. out there, we doubt we'd hear it over the jets.
The tub we got is an Arctic spa, and one neat thing about this brand is that it comes with its own base, so if you don't like where it sits, you can move it. No concrete pads all over the place if you keep moving it.
We had it set up so the cover opens to the west, and blocks the west wind while it's up.
We plan to move ours about 20 feet to the east this summer, that's all. It's staying outside.
So before spending a lot of money converting the garage, I would try it outside for a year.
We have a nice big garage too. This is where we keep our cars & junk.