A common misconception I hear from investors is “I own a short-term rental and kept my personal use below the 14-day limit, all my expenses are fully deductible right?”
Not quite.
Staying below the 14-day or 10% threshold keeps your property classified as a rental business, not a personal residence. That matters because it means your net losses are deductible against other income (assuming you meet the STR material participation requirements). But it doesn't erase personal days from the tax calculation. Any personal use at all requires you to split every expense between rental and personal use. And yes, that includes depreciation.This has real implications for cost segregation. If you're front-loading a large first-year depreciation deduction through bonus depreciation, the percentage of days you actually rented the property determines how much of that deduction you get to use. A handful of personal vacation days can quietly chip away at a five- or six-figure write-off.
This post answers two questions that come up constantly in STR tax planning:
- Does depreciation have to be prorated?
- What actually counts as a rental day in the first place?
What the 14-Day Rule Actually Does (and What It Doesn't)
The 14-day or 10% test under IRC Section 280A is the dividing line between two very different tax situations for STR owners.
Here's how it works: if your personal use stays below the greater of 14 days or 10% of the days the property was rented at a fair rental price, your property is classified as a rental business — not a personal residence. That classification gives you access to net loss deductions, and expenses aren't capped at rental income.
Go above those thresholds and the property shifts to residence status. Deductions get capped at gross rental income, losses from the property are suspended, and the tax picture looks significantly worse. (For a deeper dive on the full consequences of crossing that line, see our post on the STR loophole and when it applies.)Most active STR investors are aware of this test and manage their personal use accordingly. What's less understood is the rule underneath it.
Section 280A(e)(1) requires that for any year with both rental use and personal use, every expense must be allocated between the two purposes — even if you used the property personally for just a single day, and even if that day still leaves you far below the 14-day threshold. The 14-day test and the expense allocation rule are two separate things.
Conflating them is one of the more common mistakes STR owners make.
Yes, Depreciation Has to Be Prorated
Under the allocation rules, three categories of expenses have to be divided between rental and personal use:
Category 1 — Mortgage interest and property taxes. These are expenses that are deductible to some extent regardless of whether the property is rented. A portion goes to Schedule E against rental income, and a portion may go to Schedule A as a personal deduction. Category 2 — Operating and maintenance expenses. Utilities, insurance, repairs, cleaning fees, and similar costs are only deductible to the extent they're attributable to rental use. The personal portion is simply nondeductible. Category 3 — Depreciation. This includes all cost recovery, including accelerated depreciation from a cost segregation study. IRS Publication 527 is direct on this point: you can deduct depreciation only on the part of your property used for rental purposes.There is no exception for depreciation. If you have personal days, a rental-use percentage applies to your depreciation, the same way it applies to everything else.
What Actually Counts as a "Fair Rental Day"
This question matters both for the 14-day threshold calculation and for the allocation fraction. The answer is more specific than most people expect.
A fair rental day requires two things: the property must be actually rented and occupied, and it must be rented at a fair market rate.Days the property is listed on Airbnb or VRBO but not booked do not count. Days the property is available but empty do not count. Rental days are booked nights.
Schedule E asks you to report "days rented at fair rental price" — not days available for rent. That distinction is built into the form.
Below-market rentals count as personal use. If you let a family member stay for a heavily discounted rate, that day is treated as personal use, not a rental day. The IRS defines fair rental price as an amount comparable to what similar properties in the same area are charging. Standard Airbnb pricing and commercially reasonable dynamic pricing typically qualify, even with seasonal discounts or promotional rates. Repair and maintenance days are neutral. If you visit the property primarily to perform or oversee repairs, those days are not personal use days and are not rental days either. They effectively don't exist for purposes of the proration fraction, but they do support the deductibility of the related repair expenses.The key word is "primarily." IRS guidance focuses on the primary purpose of the day. If you spend the day managing contractors with an hour of leisure, that reads as a maintenance day. If you spend the day at the beach with an hour answering contractor texts, that reads as personal use.
"On call" doesn't count. Being available by phone while not at the property doesn't reclassify a personal day. Physical presence and primary-purpose documentation are what matter — so keep maintenance trips clearly separated from personal use, and log time, invoices, and photos when the primary purpose is business.How the Proration Formula Works
The IRS method for calculating the rental-use percentage under Section 280A(e)(1) is: Fair Rental days ÷ (Fair Rental days + Personal use days)The important detail is what goes in the denominator and what doesn't.
Vacant days are excluded. If the property is listed for rent but not actually rented, those days are not rental days and not personal days. The IRS only counts days rented at a fair rental price when calculating the rental-use percentage.If your property sat empty for 120 days while listed on Airbnb but not booked, those days don't count as rental days, but they also don't count as personal use days. They don't factor into the allocation fraction at all.
Example:- Rented and occupied: 200 days
- Personal use: 20 days
- Vacant while listed: 145 days
- Total days in year: 365
Not 200 ÷ 365 = 54%.
That distinction changes the math significantly. Investors who assume their rental percentage is calculated against the full year are often underestimating their actual deductible percentage.
The IRS Method vs. the Bolton Method
For mortgage interest and property taxes specifically, there's a competing approach with favorable Tax Court precedent.
In Bolton v. Commissioner (1981), the Tax Court approved a different allocation formula for mortgage interest and property taxes specifically: rental days ÷ 365 rather than rental days ÷ days used. The reasoning was that these expenses accrue ratably throughout the year regardless of occupancy — unlike utilities or cleaning fees.This matters because of how Section 280A(c)(5) stacks deductions when a property is classified as a residence: interest and taxes come off rental income first, then operating expenses, then depreciation. The less of your interest and taxes allocated to the rental bucket, the more room there is for depreciation to be deductible. For depreciation and operating expenses, the IRS fraction still applies under Bolton — the court method is only for interest and taxes.
Here's a simplified comparison using 90 rental days, 30 personal days, and 245 vacant days:
| Method | Mortgage Interest & Taxes | Operating Expenses & Depreciation |
|---|---|---|
| IRS Method | 90 ÷ 120 = 75% | 90 ÷ 120 = 75% |
| Bolton Method | 90 ÷ 365 = 24.7% | 90 ÷ 120 = 75% |
The IRS continues to push its method in publications and proposed regulations, and practitioners outside the Ninth Circuit carry some litigation risk applying Bolton. But it's a position with strong precedent and worth discussing with your CPA in years with significant depreciation.
Why This Hits Harder When Cost Segregation Is Involved
A cost segregation study doesn't change the proration rules, it amplifies the stakes.
Cost seg works by reclassifying components of a property into shorter depreciation lives and, with 100% bonus depreciation now available under current law, allowing much of that reclassified value to be expensed in year one. A study on a $1.5 million STR might identify $300,000–$400,000 in components eligible for immediate deduction.But that deduction is subject to the same rental-use percentage as everything else.
If your rental-use percentage is 85%, you lose 15% of that number before it ever reaches your tax return. On a $350,000 bonus depreciation deduction, that's $52,500 in deductions gone — permanently, not deferred.Here's why the timing matters: the year you complete a cost segregation study is typically the year with the largest single depreciation event of your ownership. It's also the year where the rental-use percentage has the highest dollar impact. A difference of 10 percentage points in rental-use can translate to five or six figures in lost deductions depending on the study size.
If you know you want to take a personal trip to your STR, plan around the cost seg year.What This Means for Planning
A few practical takeaways for STR owners:
Know your rental-use percentage before the year ends. Don't reconstruct it at tax time. Track rental days and personal days in real time, especially in years when you're planning significant capital investment or a cost seg study. The year you do a cost seg study is the wrong year to take personal days. The deduction is largest, and the cost of lost rental percentage is highest. If personal use is unavoidable, model the impact before placing assets in service. Distinguish maintenance trips from vacations. These days are treated differently under Section 280A, and the documentation requirement is real. Keep the two purposes clearly separated. Ask your CPA about the Bolton method. If your property has significant mortgage interest or property taxes and you're in a year where the Section 280A(c)(5) income cap is a concern, the Bolton approach for allocating interest and taxes may preserve more of your depreciation deduction. Time personal use strategically. There's nothing wrong with using your own property. Just understand that personal days have a direct cost, and that cost is highest in years with large accelerated depreciation events. If you're working through a cost segregation study for a short-term rental or trying to understand how personal use has affected your deductions in prior years, reach out to us Maven Cost Seg and we’d be happy to discuss your situation. This post is for educational purposes only and