Jim, the weather is indeed a consideration, but I don't use my music room much NOW so I don't know if that's because I hate the room (which I do) or I am just sick of music (which I am). The picture in this post is what the room would look like if there was no wall and I could shoot a photo of the shape. And this is what I hate about it.
The amount of usable space is cut into by those barn shaped angular walls. About halfway up the sloped walls is 5.5 feet, so everything below that point is barely usable room. The room I'd build would be 192 sq ft of actual usable space. Since I am limited to 200, that is as big as I can go without an expensive permit that has a long list of requirements attached to it.
These are all the things I have to weigh out (or put on the old school Ben Franklin T with the pros on the left and the cons on the right). As considerations grow, so does the price tag. I don't even have a price on the slab yet. That alone may kill the idea! I haven't had to price anything like that since I had a driveway put in 11 years ago. That was 55 feet of prep and concrete, and it came in at $2300. 11 years ago. 4 years ago I asked a guy about a 10x8 area and he wanted $4000! This would be 15x15. Then there's the headache of finding somebody I trust to do it. I have a bad opinion of contractors.
Given that this kind of thing is a toy for me and will never be used professionally, I won't get too concerned about acoustics. I don't record live instruments. The only live music that place would see is if I invited people to come by and just have fun, but I'll never record a live band.