That's an excellent question. There are many moving pieces but its not hard to ballpark the answer. Square footage values are flawed because they don't consider how the space is being used. A better method is to look for comparable homes that have the same number of bedrooms and bathrooms that your home would have when your renovations are done. These comparable sales should be similar in size (after renovation), quality of finish, style and age. Then, subtract what your home is currently worth. That should let you approximate what the extra bedroom and bath would add to your value.
Hope that helps.