The 15-day rule is the one worth knowing before anyone rents anything. Section 280A(g) says that if a dwelling unit is used by the taxpayer as a residence and is actually rented for fewer than 15 days in the taxable year, no deduction attributable to the rental is allowed and the income from that rental is not included in gross income at all. It is a complete exclusion rather than a deduction, so an owner who rents a home for a fortnight around a local event reports nothing. This is the provision widely known as the Augusta rule, and its two halves travel together: give up the deductions and the income disappears with them.
Depreciation is the deduction that makes a rental different from any other investment, and it starts earlier than owners expect. The clock begins when the property is ready and available for rent, which is the date it is listed and habitable rather than the date the first tenant signs. Residential rental property, meaning a building for which 80 percent or more of the gross rental income comes from dwelling units, is recovered over 27.5 years on the straight-line method with a mid-month convention, so the first and last years are prorated to the middle of the month. Land is never depreciated, which makes the allocation of the purchase price between land and building the single most consequential number recorded at closing. Appliances, carpeting and furniture used in the activity are a separate, much shorter class, so they are worth listing separately rather than folding into the building.
Depreciation is not free money. It reduces basis as it is claimed, and the reduction is measured by what was allowed or allowable rather than by what was actually taken, so declining to claim it does not preserve the basis. What happens at the sale, and the separate question of whether rental losses can be set against a salary, belong to the terms that own those subjects.
The repair-or-improvement line decides timing, and the timing is worth real money. A repair is deducted in the year it is paid. An improvement is capitalized and recovered over the building's life. The test in the tangible property regulations is whether the work results in a betterment of the property, a restoration of it, or an adaptation of it to a new or different use. Fixing a pre-existing defect, enlarging the property, or increasing its capacity or quality is a betterment. Replacing a substantial structural part, or rebuilding to a like-new condition, is a restoration. Converting a garage into a shop is an adaptation. Patching, painting and servicing are not. Two elections soften the edges: a de minimis safe harbor for small acquisitions and a safe harbor for routine maintenance performed more than once over a stated period.
Section 280A is a dwelling-unit rule, so it does not reach every rental. Bare land, commercial space and a purpose-built rental the owner never occupies sit outside it. What all of them share is the ordinary requirement that the activity be conducted for profit and documented as such: leases, bank records, invoices, mileage logs, and a basis schedule that survives every year the property is held.