In this market when inventory is at an all time high, and buyers have so many wonderful choices & bargains, get the inspector, get your addition up to code and permitted. Every sq ft helps with your list and ultimately sales price. It is a sq foot market!! Doing that now will solve all kinds of issues that would pop up during contract negoitations, inspections & closing. I actually have a home inspector that does this frequently. It could cost minimum of $500, and depending if walls need disturbed due to electrical work, it could cost upwards of $5000. Good Luck!
Turning a garage into a bonus room brings up some red flags due to the other "requirements" for being counted as square footage. In addition to the requirments oulined in this link http://www.ncrec.state.nc.us/publications-bulletins/sqft.html one very important and typically overlooked item is whether or not a permit was obtained to make the improvement. Because without a permit you will likely not have the CO (Certificate of Occupancy). No CO, No sq. ft.
I believe Barrett pretty well nailed it. For the most part the heating source and the permits would be the major factors. If a permit was pulled then A) an inspector would have been involved and it would have been done to code. Otherwise there generally are very few if any contractors that will inspect, pull permits and put themselves on the line to say that it is up to code in order for you to get your CO.
Hope this helps,
Buyers at times dont like seeing window units in a room addition.
If the room is heated and cooled by the wall unit, it may count as living area if other criteria are met. Go to the following link http://www.ncrec.state.nc.us/publications-bulletins/publicat and click on "Residential Square Footage Guidelines" to see a complete set of the guidelines for measuring square footage.