Does the Addition include any plumbing, or drainange or is it just four walls, will the addition need windows? A/c? Etc
In general expect to spend about $75-$100 per square foot.
Best thing to do is get 2-3 bids from a licensed contractor and factor in your drawing and permits and insurance costs etc as well.
If you need referrals to vendors, i have a list i can forward you.
That's a question that calls for quite a bit more detail. Is this a single story or two story? Will you be adding a story? Will a kitchen or bathroom(s) be in the addition? Is the lot level where the addition will go and will it be on a slab? These and many more questions can have a huge impact on pricing. My partner's husband is a licensed general contractor that does room additions and remodels. He does excellent work. I suggest you give him a call - Terry McGuire, 714-336-7072, to discuss what you have in mind. That way you'll have a much better idea how much your project will cost.