If the basement is similar to the upper levels of the home as far as finished heated/cooled space in the home, the basement possibly could be counted to be included in the total square footage .
If any of the basement is below level of ground, that portion of square footage needs to be shown as living space below level and added to the upper level square footage to total the square footage of the home. but it all needs to be finished,heated and cooled. The other element is ceiling height in the basement - at least 7ft - not lower - otherwise, the square footage cannot be counted as part of total square footage of home. I have been told over the years that basements should have access to an exterior door.