Depending on the source of the individual appraising the home, a deck is considered a ‘gray area’ in terms of determining if a deck adds square footage to the house. An all-encompassed standard states that when counting total square footage of a dwelling, a deck is included - if part of that deck has an enclosed area with heating or AC functions. This would classify that area as livable. The uncertainty lies within whether the rest of the square footage of the deck around that enclosed area will also count.
If a city, village or township-employed appraiser is making the call, it is up to what the code written regulations say. On the flip side, if an independent, subcontracted appraiser is inspecting the space, it is up to his or her judgment call.