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.
Please consult a licensed contractor with your specific specifications for them to determine the requirements to meet your specs based on current building design, electrical capacity, plumbing updates, etc.
And is this a slab or raised ground-floor addition?
Is it a second story addition?
Is construction in Napa a wee bit higher than Stockton?
I am sure that it has so many variables that your answers will be: Please contact a contractor for estimates.
Do you think?