Besides being one of the things the IRS looks at closely -- thereby increasing your chances of being audited -- you can only legally deduct a percentage of the mortgage interest.
**I am not a tax professional** but unless the tax laws have changed, you need to calculate how many square feet your office is, and divide it by the total square footage of the whole house to determine what percentage you can deduct.
Home office = 100 sq ft.
Whole house = 2000 sq ft.
100/2000 = 0.05 or 5%
So, if you paid $10,000 in mortgage interest, you can only deduct $500 as a business expense. You'll have to decide if $500 is worth an audit.