First, be sure that you qualify for a home office. You should be self employed and use the space regularly and exclusively for your business activities. If you use your den and someone also watches TV in it, it doesn’t qualify.
yes, only the interest portion of your mortgage payment can be deducted. You can deduct 90% of that interest and 90% of property tax on Sch A if you itemize and 10% of those expenses as a business expense on Sch C.
There is also a simplified office deduction available to you which you can read about here:
https://www.irs.gov/businesses/small-businesses-self-employed/simplified-option-for-home-office-dedu...
