Tax deduction of finished below-grade basement

[Rick Im]

A 2,000 sq. ft. single-family home with a 1,600 sq. ft. finished below-grade basement is currently on the market for sale. I am considering using the entire basement as my new home office (given my job, I need a large space). I wonder if I could deduct mortgage interest based on the basement's proportion to the entire house (1,600/(1,600+2,000)). I am asking because the below-grade basement is not counted toward living space in Kentucky, even if it is finished, and I was told that appraisers count only half of the basement space when valuing the property.