It wont be cheap. Expansion that includes footings to roof is clearly the most expensive way to add a room. That is not necessarily a bad thing however if you plan to use the room extensively. Key to value, hire an architect. Yes, that's right. They will save many hours of work, as well as can oversee the work to make sure you are getting what you are paying for. Another Key to value is to not overbuild your neighborhood. The finishes you place in this room should match your home as well as those homes in your neighborhood. Remember, anything you do to your home that is not typical to your neighborhood will more than likely NOT be returned to you upon sale.
Oh yea, you asked what the costs will run.....depends on your contractor....and your finishes but plan on $70-$100/sq.ft
Enjoy the process, work with professionals. Cheap out and you will more than likely be sorry.
Keller Williams Realty Nashville