Mine is 26x30 stick built, used 2 gluelam beams under the full dormer walls and 2x12 joist. I built a dormer on each side of the roof (11/12 pitch gable) that gives me 8' ceilings in the dormer. The dormer walls are full length and sit on top of the gluelams for support. the dormers only give about 4'x8' more floor space on each side, but the ceiling space adds about 8'x9' per dormer.
I could not build the barn style due to covenants, but they are great bang for your buck.
I built my knee walls 4' from outside wall and doubled the 2x12's under the knee walls. I would really consider the dormers. Since I had the space I turned one into a closet and one into a bathroom. Remember to design room for a good size staircase. I thought I built mine too wide (based on framer recommendation)until I started hauling materials and furniture upstairs.
Whether you plan to right away or not, rough plumb for a bathroom. I rough plumbed mine (one upstairs and one under the staircase), and now have a toilet in both. The bonus room has turned into an apartment of sorts. Great for a hang out spot/extra sleeping quarters etc.
It does not cost much more to add rough in specs to the slab. I added water line and empty pvc pipes that I have since filled with cable and CAT5 wiring.
Good luck!
