Ron offers up some pretty sound advice. There are indeed a lot of variables to your question. Before I'd contact even a contractor and waste a lot of their time I'd first do down to the building and planning department and find out what their requirements are.
Then I'd contact an architect or an architectural draftsman at the very least as a 12x12 would hardly capture the interest and/or attention of a full blown architect. You will have to have a plan before you start wasting a contractors time unless the contractor could design it for you which they often can on very small projects.
Once you have a plan a contractor can then proceed to price it out. If you are talking a one-story level without any plumbing or kitchen features, say a bedroom or any study and it was a slab on grade vs a raised floor you'd probably be in or around 100 to 150 per sf. If, however, it was a bathroom you'd probably be looking at around 200 per sf or a kitchen addition you'd probably be looking at about 250/sf,
You can add on about 25 per sf for 2nd story additions and 50 per sf for a raised floor with a concrete perimeter wall. All that stated that would be San Diego price points.