I built a 12x16 when I moved into our current house in Central Texas.
I used concrete blocks of different sized and gravel to level out piers
for a base for the floor joist deck of 2 x 8's. I used a tongue and
grove waferboard underlayment for flooring and I would not recommend the
waferboard part. It flakes up too easily.
I made the wall height a full 8 feet and pitched the roof at 12/12 for a
loft effect. I had to switch to metal roofing from the normal plywood
deck and shingles due to the dramatic pitch of the roof. But it turned
out good since I could do the metal roofing one strip at a time by
standing on the ceiling joists while doing it with a friend.
I built my own double doors out of 2 x 4's and siding that matched the
hardboard siding I used on the outside.
The only thing I would do different would (besides the flooring)be to do
a front upper door to the loft so I could put in long pieces of wood or
pipe if need be.
IF I chose flooring today I would probably use plywood decking or if I
wanted a cadillac floor, 2 by tongue and groove.
Good luck and have fun doing it.