One Property, Several Uses: How the Deductions Split
How to allocate costs when a property is part home, part rental, or part office, the day-count and square-footage methods, and what happens at sale.
Reviewed by the Fairlight Accounting cross-border tax team — U.S. & Canadian Tax Desks
When one property serves more than one purpose — a home with an office, a rented unit, a vacation house rented part-year — expenses are split by a reasonable method: square footage for shared space, days for shared time. The business or rental share is deducted; the personal share is not, except interest and taxes under personal rules.
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Which method applies?
| Situation | Allocation method | Where deducted |
|---|---|---|
| Home office in your residence | Square footage of the office over the home (or the simplified method) | Schedule C or reimbursed by your corporation |
| Duplex: you live in one unit, rent the other | Square footage (or units if equal); shared costs split | Schedule E for the rental share |
| Room rented in your home | Square footage, times days rented if not all year | Schedule E |
| Vacation home rented part of the year | Rental days over total days used | Schedule E, with limits |
| Short-term rental with personal use | Rental days over total days used | Schedule E or C, with limits |
Land and the personal share of the building are not depreciable; the rental or office share is.
What are the vacation home limits?
If you use a rental property personally for more than the greater of 14 days or 10 percent of the days it is rented at a fair rental price, you use it as a home for the year (Section 280A(d)(1)). Expenses are allocated by rental days over total days used, and if the rental shows a loss, the rental deductions cannot exceed rental income — they are taken in a set order (the rental share of mortgage interest, taxes, and casualty losses, plus direct rental costs such as advertising, first; then operating costs; then depreciation), with the excess carried forward. If you use the property as a home and rent it for fewer than 15 days in the year, the rent is not reported and no rental expenses are deducted (Section 280A(g)).
How does a home office interact with the rest of the home?
The office must be used regularly and exclusively for business and be the principal place of business or a place where you meet clients or customers in the normal course of business. Under the actual-expense method, the deduction is limited to the business's net income, with carryforward of the excess (the simplified method allows no carryforward). Depreciation claimed on the office portion is recaptured at sale, and the office share of the gain is still eligible for the home sale exclusion if it is within the same dwelling unit.
What happens when the property is sold?
The residence share of gain qualifies for the home sale exclusion if you owned and lived in the home for at least two of the five years before the sale; gain equal to depreciation taken after May 6, 1997, cannot be excluded and is taxed at up to 25 percent as unrecaptured Section 1250 gain; and a rental unit separate from your dwelling unit (like the other half of a duplex) is a separate asset — its share of the gain is not excludable, though a like-kind exchange can defer it. Converting a home to a rental sets the depreciable basis at the lower of adjusted basis or fair market value, and converting a rental into a home can reduce the exclusion for periods of nonqualified use after 2008.
Frequently asked questions
Can I deduct the whole mortgage if half the house is rented?
No. Half the interest goes to Schedule E as a rental expense; the other half is personal mortgage interest, deductible only if you itemize and within the personal limits.
Do days I spend fixing the property count as personal use?
Not if you spend the day working substantially full time repairing and maintaining (not improving) the property — even if family members use it for recreation that day.
Does renting to a relative count as rental use?
Only if the relative pays fair rent and uses it as a main home; otherwise those days are personal use.
Can I use the home office simplified method on a rental?
The simplified method is for home offices only ($5 per square foot up to 300 square feet); rentals use actual expense allocation.
Official sources
The IRS explains: “In general, your rental expenses will be no more than your total expenses multiplied by a fraction, the denominator of which is the total number of days the dwelling unit is used and the numerator of which is the total number of days actually rented at a fair rental price.” — Internal Revenue Service, Publication 527 (2025), Residential Rental Property, https://www.irs.gov/publications/p527
The IRS explains: “If you were entitled to deduct depreciation on the part of your home used for business, you cannot exclude the part of the gain equal to any depreciation you deducted (or could have deducted) for periods after May 6, 1997.” — Internal Revenue Service, Publication 587 (2025), Business Use of Your Home, https://www.irs.gov/publications/p587
Next step
Fairlight Accounting handles U.S. domestic, cross-border (U.S.–Canada), and international tax returns, plus bookkeeping, payroll, and CFO advisory. Our U.S. Tax Desk sets the allocation once, documents it, and carries it through to the sale. See pricing or book a free fit call.
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