Your partner is probably sharing living expenses as opposed to having a landlord/tenant relationship.
You need to prorate the square footage of the apartment with the rest of the house. You would divide the mortgage interest and property taxes in that proration on both Schedule A for your personal portion and Schedule E for the rental portion.
If the apartment is not a true rental situation, meaning you do not charge the full rental value of the apartment to your son, then you have a not-for-profit rental. That is treated differently.
Not Rented for Profit
If you don’t rent your property to make a profit, you can’t deduct rental expenses in excess of the amount of your rental income. You can’t deduct a loss or carry forward to the next year any rental expenses that are more than your rental income for the year.
Where to report.
Report your not-for-profit rental income on Schedule 1 (Form 1040), line 8. If you itemize your deductions, include your mortgage interest and mortgage insurance premiums (if you use the property as your main home or second home), real estate taxes, and casualty losses from your not-for-profit rental activity when figuring the amount you can deduct on Schedule A.
