I currently have a ranch-style condo listed that has a 4 season sun room with tiled floor, heated and air conditioned, just like yours. it is accessed from the dining room.
It is included as part of the town assessment for tax purposes, too, so they do factor in the increased sq. footage. Although we don't make use of square footage in our listings on most resales, if I were to calculate it, I would count the sun room as part of the total.