The answer to your question depends alot on the location. I know that the "Location, Location, Location" as being the 3 most important things in real estate is used as a joke, but it really is true. What type of neighborhood are you in and where is it located. What are other houses of the same size as your current house selling for in you neighborhood? How much are houses with the square footage that your house will be after the addition selling for? These are 2 major questions.
Before this market slow down, how was your neighborhood appreciating?
Do you plan to sell the house immediately or do you plan on enjoying the space for a while first? All of these questions need to be researched before an informed answer can be given. If you will let me know what subdivision or area the house is in, I'll see what I can find out.
Let me know how I can be of service to you.