The IRS days-of-use allocation of all expenses.
Sec. 280A(e)(2), the IRS interprets Sec. 280A(e) to mean that all deductible expenses related to a property are allocated between fair rental and personal use days based on the ratio of fair rental days to the total number of days used, not just to those expenses that are deductible only in relation to the rental of the property.
Turbotax use the IRS method to allocate expenses on a vacation rental.
If you’re looking to claim the most tax breaks possible, you need to carefully track the amount of time you and your family spend at your vacation home.
If you personally use your property and sometimes rent it to others, special rules apply. You must divide your expenses between the rental use and the personal use. The number of days used for each purpose determines how to divide your costs.
Renting Your Vacation Home
https://www.irs.gov/uac/newsroom/renting-your-vacation-home-1
To figure your deductible rental expenses and any carryover to next year, use Worksheet 5-1.
Personal Use of Dwelling Unit (Including Vacation Home)
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