Typically for an appraisor to value it as finished space not only does it have to be heated, but also it must be finished. I had a listing a couple of years ago with the same situation. The area was heated but not completed. It had concrete floors and unfinished drywall and the appraisor would not count it as space. The public viewed the space as unfinished as well. When selling a home, the opinion that truly matters is the public. How will they view your space? Will they look at it as expansion area or another finished area of your home. They will be the ultimate decision maker in deciding your home's value. Furthermore, if you do sell your home and then the appraisor won't valuate the space as finished, it may cause you a problem before closing. Put yourself in your buyer's shoes. How would you view the space if you were buying this home? Sometimes it is hard to be objective about your own home, but if you can remove your personal feelings and look at the space objectively, you should find your answer.
If you need help selling your home, give me a call.