The quotes can vary as Colin mentions, below. You should check with the County, if you havent' aleady, to find out if you can even put an addition that size, on the house you have in mind. They have stricter rules now about such things based upon lot size.
My recommendation is that you write a Statement of Work (SOW) that details what you want done and THEN have 3 contractors come and talk with you and then prepare proposals/quotes for you based upon your SOW. That way you get no misunderstandings about the work (surprises later) and everyone works from the same baseline descriptions (Apples to Apples so to speak). The price /sq ft depends on what materials you are using for the finish work. There will be a low, mid, high price depending on what you use; esp. for kitchens and baths because of electrical, plumbing and probably a "heavy up" on your electrical panel. There are a lot of factors to consider and you should follow my recommended process:
Initiate - Plan - Execute - Close
Many people want to start Executing before they get all their ducks in a row, select products, and really think through things. That causes problems, work stops, and other problems.. Save yourself headaches and do it right the first time.
I can recommend several contractors to you all of which I've worked with for several years.
Check out http://www.designs4less.biz for mor information
The two contractors I recommend are:
New Leaf Construction: - Daniel - http://www.newleafconstruction.com - 703.489.9978
Davis Construction: - Aaron - 703.867.3268