Not long ago I had a listing which had a beautiful 3 season porch and the Seller went out and bought two very nice electric baseboard heaters and had them installed (hardwired) in the porch......presto, a very nice 4 season porch which we were able to count as livable sq. footage. I believe it added another almost 200 sq. ft. and it became a great selling feature of the home.
Thanks, Todd Norsted
I am not in your area, so this may not be exactly right.
Living area needs to be permitted by the local government. Typically one of the standards used is heating, however there are other standards as well, particularly fire codes. The best way to remdy the problem is to ask the local building inspector or Dept. of Building and Safety.
If you looking at listing your property, discuss with your Realtor. Better to offer it as "bonus space", disclosing the estimated square footage, rather than including it in the square footage if you do not have permits.
Disclaimer - local customs may reveal something different.