It looks like you're in Pittsburgh but are asking this question in Oakland CA. Where are you planning on doing this construction? This will greatly change the price of your renovation.
The cost will also depend a lot on finishes, structural work required (support beams/posts, new or updated foundation work etc) and the type of home you have, among other factors.
I work on a lot of renovations in the bay area and am in charge of budgeting and project management for an LLC that does such renovations. If you plan on using high-end finishes (hardwood floors, any natural stone or more expensive tiles etc), you'd be looking at at least $350/ft in the bay area. That doesn't include non-construction costs like your architect, structural engineer, and permit fees. That number can easily go as high as $450/ft for really high end finishes.
If you're doing the work as cheap as possible, I think you could probably get this addition done at $250/ft for construction costs, but I doubt it would be much cheaper than that.
Of course, a contract will be able to give you an accurate quote. If you'd like help finding one (or an architect, landscaper, structural engineer etc), I'd be more than happy to give you some recommendations.
Hope this helps!
About $117 per square foot.
Should call your Insurance company and ask what Builder would they use in case the house burned down or they can recommend.
Hope this helps.