Hi there. You should include the amount of rooms on your property assessment. So, If the basement is finished, I would include it in the total rooms. Hope this helps.
To calculate the percentage of work-space-in-the-home expenses you can deduct, use a reasonable basis, such as the area of the work space divided by the total finished area (including hallways, bathrooms, kitchens, etc.). So you can include the basement rooms, if they are finished and assessed.