DH is right that this is a complicated remodel that needs consideration for many different aspects. A contractor or designer may be able to come up with suitable plans for a permit, but unless you have extreme confidence intheir abilities, it may be better to go the architect/engineer route. If you decide to try it yourself, I would at least recommend paying for an engineers time to make sure your plans are viable- it may be required. Don't count on mere issuance of a permit ensuring that the plans are good. If you decide to go the unpermitted route and accept the liability for both yourself and future owners of the property (possibly having to tear your work out later if gov't officials find out or be sued by someone you sell to later being hurt physically or financially), here are some
things you must consider:
1. The angled supports cut right into the space I'd want the room. So is there some other way to replace them? Like if I for instance framed my new room so the top edge become one of the supports?
It depends on where the supports or future walls would be in relation to load-bearing
walls below and the effect on changing rafter spans above.
2. I wonder if the floor of this attic would techically be coded to have a room. Like maybe it wouldn't support the weight. However, the room will be above another room/tall wall below - not over a big void on the second floor.
The floor of the attic supported by the floor below's ceiling joists probably would not be up to safely supporting another floor because the attic wasn't designed as living space in the first place (evidenced by insulation on the floor. They would probably have to be beefed up - thus the architect or engineer coming in handy. You could figure out what needs to be done by using span tables in the local code with attention again to load-bearing walls below and joist spans. You might add more or bigger joists.
3. We're in Georgia - and this attic gets really hot. And the "door" to this room would be in its floor - and it would be the highest point in the house. I'll tap into a duct to bring in heat and AC. But I'm wondering if this room could become impossible to temperature control. All the heat of the home would go up into it. And any cold (like AC) would fall out of it.
You would probably want to take the insulation out of the floor and add some (a lot - including infrared barrier) to the attic ceiling/roof. Some insulation could be left in the floor for sound control or if you will only heat/cool the attic room some of the time. You must investigate roof venting and vapor control for your area. Any change in insulation and use will alter these things and could cause mold or other problems. Maybe consider rerouting ducts to help with the heat and cold stratification.
I think code requires a staircase to livable space, and code requires egress windows of certain sizes for emergency escape, 50% of ceiling height must be > 7' 6" and none < 5'.
These are just some of the things to consider. I really like the idea of professional help. For the house design that is.