In my experience as a residential designer and licensed general contractor, single story additions over a raised floor system run from $85-$125/SF. A concrete slab on grade per square foot cost would range in the 10%-20% less respectively. It's all dependent on the level of finish you're looking for.
If you need any further information or some help with the design and permit documents, please visit my site at http://www.davidorozcoassoc.com. Consultations are always free and I always time for your leads.
David Orozco Associates
A good contractor that I recommend is Martin Grove.
The telephone number for Mr. Grove is: (650)464-6099.