A 2 story house with each floor having 1000 square feet has a 1000 square foot basement.
Is the square footage estimate for the house 2000 square feet or 3000 square feet?
If the basement is finished in the same quality as the area above grade and with 2 means of ingress and egress in all habitable areas (excluding baths) then, in most locales, it can be considered heated living space (area) under roof and the “actual” square footage of the basement is included in the “actual” overall square footage.
For estimating cost, this is always accepted.
But, for selling a house the house (if you ever do) you may have to separate the basement square footage from the rest of the house.
A local Realtor can advise you.
See my post: What is Included in the Square Footage of a Home