Mixed Personal and Business Use: How to Split STR Expenses

The Core Rule: Only Business-Use Portions Are Deductible

IRC §262 is unambiguous: "no deduction shall be allowed for personal, living, or family expenses." This means that if you use your phone 60% for STR business and 40% for personal calls, you may deduct 60% of the phone bill — not the full amount.

This rule applies to every expense that crosses the personal/business line: your home internet, your vehicle, your primary residence (if you rent a room or use a home office), and any property you use personally for part of the year. The allocation does not need to be complicated, but it must be supportable with documentation.

The Two Allocation Methods

Two primary methods are used to allocate mixed-use expenses. Neither is universally correct — the right choice depends on what is being allocated and what produces the most accurate result for your situation.

Method 1: The Days Method

The days method allocates based on time: rental days divided by total days the property was available or in use. This method is most appropriate for whole-property expenses where the cost varies with occupancy or time — utilities, insurance, mortgage interest (for Schedule A/E crossover), and depreciation.

Method 2: The Room Method (Square Footage)

The room method allocates based on space: rental square footage divided by total property square footage. This is most accurate for shared-structure properties — a primary residence where you rent one bedroom, a duplex where you occupy one unit, or a property where a portion is used as a home office for the STR business.

Applying the Right Method to Each Expense Type

You are not required to use the same method for all expenses. The IRS wants you to use the most accurate method for each cost category. Here is how to approach the most common mixed-use expenses for STR hosts.

Expense Type	Best Method	Notes
Electricity & gas	Days method	Cost is roughly proportional to occupancy time
Internet (whole property)	Days or room method	If renting a portion, room method; if full property part-time, days method
Cell phone	Usage time estimate	Estimate business-call percentage; 40–70% is common for active hosts
Vehicle	Business mileage log	Only actual business miles deductible; personal commute never qualifies
Mortgage interest (shared property)	Room method	Rental portion may flow to Schedule E; personal portion to Schedule A
Property insurance	Days or room method	Use same method as depreciation for consistency
Depreciation (shared property)	Room method × days method	Both space and time allocation may apply for partial-year, partial-space rentals
Home office supplies	Actual use	If supplies are exclusively for STR management, 100% deductible

Phone and Internet: Usage-Based Allocation

Your cell phone and home internet are likely mixed-use every month. The IRS expects you to make a reasonable estimate of business use and apply it consistently.

For a cell phone, track for one month the approximate breakdown of calls and data: reservations, guest messages, vendor coordination, booking platform access, and property management apps are business use. Personal calls, social media, streaming, and navigation for non-business trips are personal. Once you have a reasonable estimate (say, 55% business), apply that percentage to the annual phone bill.

For internet, the question is simpler if the STR occupies a separate unit — use the room method for the square footage of that unit. If you manage the STR from home but the property is elsewhere, estimate the hours per week you use the internet for STR business versus personal and apply that percentage.

Vehicle: Mileage Is the Only Accurate Measure

For vehicles, neither the days method nor the room method applies. The IRS requires a mileage-based allocation. You may deduct only the miles driven for STR business purposes:

• Trips to the property for turnovers, repairs, or inspections
• Trips to purchase supplies for the rental
• Trips to meet vendors (plumbers, electricians, contractors)
• Trips to banks, accountants, or attorneys specifically for STR business

Personal commuting, grocery runs, and personal errands are never deductible — even if you stop by the rental property on the way. The standard mileage rate for 2026 is set by the IRS each year; alternatively, you can deduct actual vehicle expenses multiplied by the business-use percentage (business miles ÷ total miles).

A mileage log is non-negotiable. The IRS will disallow vehicle deductions without one. Log date, destination, purpose, and miles for every business trip.

Home Expenses for Partially Rented Properties

If you rent a room or attached unit within your primary residence, you face a combined allocation: only the rental portion of the property is deductible, and you may only claim expenses for the rental days, not personal-use days.

The standard approach is to apply the room method to identify the rental space percentage, then for expenses tied to time (utilities), apply the days method on top. This double allocation ensures you are never deducting personal costs.

Consistency Is Required — But You Can Choose the Best Method Per Category

Once you choose a method for a given expense category, use it consistently from year to year. Switching methods without a documented reason creates inconsistency that can attract IRS attention. However, you are free to choose the most accurate method for each type of expense — you do not have to use days method for everything simply because you used it for utilities.

Document your methodology in writing — a simple spreadsheet or notes file that records: the expense category, the allocation method chosen, the basis for the percentage, and the resulting deductible amount. This documentation does not need to be filed with your return, but it must be available if you receive an audit notice.

Expenses That Are 100% Deductible Without Allocation

Not every STR expense requires allocation. Costs that are exclusively for the rental business — not used personally at all — are fully deductible:

• Turnover cleaning services
• Guest amenities (toiletries, coffee, welcome basket items)
• Listing platform fees (Airbnb service fees, VRBO subscription)
• Locks, key boxes, and smart-home devices installed exclusively for the rental
• Insurance specifically for the rental (short-term rental rider)
• Accounting and tax preparation fees attributable to the rental

The test is simple: if you would not spend the money but for the rental business, and you do not personally benefit from the expense, claim 100%. Keep receipts that clearly show the item or service was for the rental property.