Part of your equation will be how much to "invest" if you're looking forward to resale. My attached build was ~25% of the existing value of the house at the time. I do expect to get most of that back when we sell eventually, although it will be in inflated $. Probably not wise to spend more than that. I did go a "bit" overboard by having the second story habitable, which added a good 15% or more the overall cost.
I was the general contractor on my project meaning I pulled the permits. I hired a builder to oversee and use his subs, and I paid as we went along. Being as it was 2008, I had no trouble at all getting workers. Dumpster and porta-potty in the driveway for several months was a bit of a downer. Lesson learned - don't let workers dump their lunch garbage in the dumpster.
I did some of the work, mainly drilling studs, pulling wire, wiring outlets, and installing all the lighting and garage door openers. My wife did all the interior paint. I'd estimate our work as about 15-20% savings.
My slab at 25x30 is only slightly larger than yours, but since there's a stairway at the back the downstairs space is similar. 4" thick slab is plenty for a lift.
I needed to pay an architect for plans in order to satisfy the HOA, and the front had to be brick to match the house. Other three sides are Hardiboard.
You're welcome to come over and look at mine if you want some ideas.