You can locate qualified local contractors by going to http://www.servicemagic.com/category.Additions-Remodels.1020
I can also recommend:
Watermark Builders, Inc.
925-575-0600 - cell
That stated, if the GC has the talent and experience to do the drawings for you that would be great but expect to pay an additional fee for his/her creative and intellectual contribution. Most GC's usually will not give you estimates without some graphic reference to work with.
When someone contacts me for a price on a project the first thing I ask them for are approved drawings. Contractors are infamous for coming up with ballpark pricing unless they have some accurate plans and specs to do an accurate take off with.
Additionally, you will have to present plans and specs to he buildig department for approval before they will allow permitting. You would also be wise to request a "pre inspection" from the building department before proceeding or spending any money whatsoever.
A pre inspection will generlally cost less than $100 but you will get invaluable info right from the horses mouth about what can and cannot be done on your property. Soil and terrain conditions can adversely affect and impact any addition no matter how big or how small. Good luck.
You can also check out Online Remodeling's Cost vs Value to get an idea of the costs of certain projects