Last sentence in 220.12:
"For dwelling units, the calculated floor area shall not include open porches, garages, or unused or unfinished spaces not adaptable for future use."
Half of 2100 is 1050 so 3150 total square feet.
But I think your main question here is do you count the unfinished space, and I think the answer is you do count it if it is adaptable for future use. It also doesn't clarify what it means by future use, a storage or mechanical room is still usage but not the same usage as a living room or a bedroom.
