Pouring Foundation Under Existing Room

So long story short, the people who owned the home before me had a deck. They converted the deck to a partially finished sunroom that looked like crap, so I decided to remodel it and make it a nursery for our second little one due in March. It's held up right now by (what look like) 3 4x12 posts....no concrete...just straight into the earth. I plan to dig around the exterior of the room, and pour an actual foundation. Then use cinder blocks to go up to the sill, and then use rock fascia over the cinder blocks. Surprisingly, the room is level. It hasn't sunk at all due to the poor foundation (maybe because it's only 90 sq ft) I live in Charlotte, NC, so I had a few questions. How deep should I dig the trench for the footings? I know up north it's a minimum of 42", but have no idea what it is down here in the south. Do I need to reinforce the concrete with anything? I plan to just leave the posts in place, slightly jack up the room to get the cinder blocks under there and then put it back down.