Does replacing all the windows in a rental property count as a capital improvement?

Often, no - replacing all the original windows in a rental property with a similar but slightly better modern equivalent is usually still treated as a repair, not a capital improvement, as long as it simply restores the function the windows already provided rather than adding significant new value to the property. Modern windows being marginally more efficient or better sealed than decades-old originals doesn't automatically turn the job into a capital expense.

Where it does tip toward capital is if the window replacement is part of a larger renovation project, or if the upgrade goes well beyond restoring existing function, for example significantly changing the size or style of the windows, or being bundled into work that substantially improves the property beyond its original condition. This is a genuinely common source of confusion, because people assume that replacing "all" of something, simply because it's the whole set rather than one or two units, automatically makes it capital. Scope in terms of scale isn't the deciding factor on its own - what the replacement actually achieves relative to the property's original condition, and whether it's standalone or part of a bigger project, is what matters most.