I thought the only expenses you can usually add to basis are "capital improvements", in other words not repairs...?
For the most part, yes. But for every rule, there is an exception.
For example, if you're selling a property such as your primary residence, and the buyer wants you to replace a broken door knob to which you agree. Technically, that's a repair. However, because it was a condition of the sale, the cost of replacing that broken door knob can be added to the cost basis of the property.
Whereas when you are converting your primary residence to a rental and need to replace a broken door knob on the front door that will not lock for example, that is a repair expense. Since you did the repair before you made the property available to rent, it's just flat out not deductible at all.
On that same front, if you replaced the carpet in the house after you moved out and before it was available for rent, that is a property improvement that can be added to the cost basis of the property when you convert it to a rental.
