I had a 100,000 BTU furnace replaced with a 75,000 BTU 80% furnace. I have a two story house with a finished basement and was told the basement square footage is not used in calculating the square footage for sizing the furnace BTU's. I'm wondering if this is true? The two stories is approximately 1500 square feet and the basement would add another 500 to 700 square feet depending on if I use all the basement or just the finished area.

Did they do a manual J calculation? It is the only way to know for sure. Anything else is guess work.