If the new space is the second floor, and you are effectively changing the house from a ranch to a 2story, you could look at the value of sales of similar homes to figure out value differences. It sounds like you are going to add value, and that you will not lose here. BUT construction costs generally in the NY/CT area (not sure where Indian Land is) are still running ahead of home values, obviously if you can do it for less, with same result, that is a good way. But don't cheap out on your finishes, as every buyer after you will object. Be sure to keep finishes the same or better than what you already have.