Have you checked with the City? Is it allowed? Will you have to do additional site work to add parking?
Will it require sprinklers?
What material for outside walls? Masonry, steel stud and stucco, other?
What type of roof?
Are you including any light and/or power in your definition of shell?
How about a bathroom or just extension of plumbing?
Is the addition to the front, side or rear of the building?
The entire building will probably have to be brought up to code, do you also include that in the price?
Give me a call, I can work design/build with you to minimize your cost.