When sizing a furnace, it's important to consider the entire conditioned space that the furnace will be heating. This typically includes all above-grade living areas as well as basements, if they are finished and intended to be heated.
If your basement is a living space and is intended to be heated, then it should be included in the calculations for sizing the furnace. This is because the furnace needs to be capable of heating the entire volume of space that you intend to keep warm.
On the other hand, if your basement is unfinished or not intended to be heated, you may not need to include it in the sizing calculations. In such cases, it's essential to communicate your specific intentions and usage patterns to the HVAC (Heating, Ventilation, and Air Conditioning) professional in WMP Home who is helping you size and select the furnace. They can provide guidance based on your specific situation and needs.