Darrell's comment is right on the money. Speaking as an architect, I'd also add that an architect could help you with this process. A good architect should be able to come up with a conceptual strategy for the addition (including consideration of code, structural, and design issues), and give you an opinion of probable construction costs.
AIA Seattle has an excellent online resource for finding a local architect: http://folio.aiaseattle.org/residential
And if you're interested in modern design, check out our website: http://eggfarkarch.com
I hope that helps.
See if any of these provide you some guidance - I linked you straight into the attic remodel:. http://www.remodeling.hw.net/2011/costvsvalue/article/atticb
You have read several answers already, so I will refrain from adding details should you have moved on with your needs. If you still have any unanswered questions and care to have an informational telephone conversation with a builder client of mine, I know she would be happy to spend a few minutes with you. She works exclusively on her own properties and projects, and has for well over 10 years, so you need not be concerned about a sales pitch. Instead, she can likely provide you with some initial direction and a rough idea of your cost categories.
Whatever your plans - Good luck!
Coldwell Banker Danforth & Associates
206 718 0703
Take care, Jirius Isaac
The best way to get an accurate cost for your 2nd story addition would be to take your plans and specifications to 2 or 3 licensed general contractors. All reputable contractors will give you a FREE estimate for the addition.
I would do you a tremendous disservice by rattling off a price range with so many variables, lack of knowledge of your requirements upstairs (2 bedrooms and a game room, or a bathroom, or you trying to put in a rentable apartment with a kitchen, etc) and lack of current structural, mechanical and electrical component knowledge. I would shop around, get recommendations and definitely get at least 2 or 3 estimates before committing to a contractor. Good luck.