We started offering scheduled maintenance on completed projects a year ago, partly as a service and partly to find out what actually fails after handover.
The answer, overwhelmingly, is water and sealant. Not structure, not finishes, not the things anyone worries about during a build. Almost every callout traced back to a junction that needed re-sealing eighteen months in.
That has changed how we detail wet areas and external junctions on new work, which is the whole argument for maintaining what you build: the feedback loop closes on your own drawings.