I am going to build an add on of 600 sq. ft. Is it possible if I were to take the blue prints, would you be able to tell me an estimate of the cost to build it?

Figure $200 a square foot

Sounds like you need to hire a someone that knows construction basics.

Do not take in your Blueprints they could be damaged....they can estimate..without them....add 20-30% more for unforseen expenses (miscellaneous)

I don't think it is a good idea. The associates at Home Depot are not trained to do that. You have to see what materials you are using and just add them up, then add another 20 percent.