If you use the dwelling unit for both rental and personal purposes, you generally must divide your total expenses between the rental use and the personal use based on the number of days used for each purpose. You won't be able to deduct your rental expense in excess of the gross rental income limitation (your gross rental income less the rental portion of mortgage interest, real estate taxes, and casualty losses, and rental expenses like realtors' fees and advertising costs). However, you may be able to carry forward some of these rental expenses to the next year, subject to the gross rental income limitation for that year. If you itemize your deductions on Form 1040, Schedule A.pdf, Itemized Deductions, you may still be able to deduct your personal portion of mortgage interest, property taxes, and casualty losses from Federally declared disasters on that schedule.
For more information: https://www.irs.gov/taxtopics/tc415
