A capital improvement is work that betters, restores, or adapts your rental property, so the cost is added to basis and recovered through depreciation over years rather than deducted in full the year you pay it. That is the line that separates it from a repair, which keeps the property in ordinary working order and comes off your income in the same tax year.
The IRS uses a three-part screen people call the BAR test: a cost is capital if it is a Betterment (it fixes a defect, enlarges, or upgrades the property), an Adaptation (it puts the property to a new use), or a Restoration (it rebuilds a major component or returns the property to working order after it had deteriorated). Pass any one of the three and the cost is an improvement, not a deduction you take now.
In practice
Say a unit needs work between tenants. You patch a section of drywall and repaint for $900. That keeps the place in the condition it was already in, so it is a repair and the full $900 comes off this year's income. Now say you tear out the original kitchen and install new cabinets, countertops, and a layout for $14,000. That is a betterment, so it is a capital improvement.
The $14,000 does not vanish. It gets added to your basis and depreciated. Residential rental improvements use straight-line depreciation over 27.5 years with a mid-month convention, so a rough full-year figure is $14,000 ÷ 27.5, which is about $509 a year. You trade a large one-time deduction for a small one that repeats for decades. The repaint gave you $900 this year; the kitchen gives you roughly $509 a year for 27.5 years.
Why it matters to a small landlord
Misclassifying these is one of the easiest ways to overstate or understate a Schedule E. Call a true improvement a repair and you have taken a deduction you were not owed; call a repair an improvement and you have buried a write-off in a 27.5-year schedule for no reason. The mechanics of that schedule are worth knowing cold, which is why it helps to read how rental property depreciation actually works before tax season, and to see where each kind of cost lands when you fill out Schedule E line by line. The edge cases, a partial roof versus a full one, a furnace versus a whole HVAC system, are where a CPA earns the fee.
A capital improvement is one flavor of capital expenditure, the broader bucket of money spent on long-lived assets. It raises your cost basis, which matters again the day you sell and figure your gain. And before you capitalize a small item out of habit, check whether it fits under the de minimis safe harbor, which lets you expense qualifying purchases up to $2,500 per invoice instead. Get the classification right once and the rest of the year's numbers follow.