When a home is rented for fewer than 14 days during the tax year, the home is considered a personal residence. Mortgage interest and real estate taxes may be deducted as itemized deductions on Schedule A, and the owner is not required to report rental income.
When you rent a home to a relative, such as a spouse, child, grandchild, parent, grandparent, or sibling, any day rented at less than the fair rental price is considered a personal use day. To avoid having the rental days considered personal days, the property must be rented at fair market rates and be the renter's principal residence.
The issue, in this case, is that the parents want to offer dear daughter a bargain, charging her less than fair rental value. If they go this route, they will have to allocate expenses between personal and rental expenses. All of the days the home is rented to the daughter at less than fair rental value are considered personal days, so the rental portion is zero. They would have to include all of the rental income received from their daughter in taxable income, but none of the rental expenses would be deductible, other than mortgage interest and real estate taxes, which would be deductible as itemized deductions on Schedule A.