The building ones, where the prevented repairs are the expensive ones.
Clear the gutters and check the downpipes, twice a year, autumn and spring. Overflowing gutters put water down the wall, and that becomes damp, then plaster, then in the worst case the wall itself. This is the highest-value item on any home maintenance list and it is an hour.
Check the sealant around the bath and shower. Any gap, any lifting, any black. Water getting behind that goes into the floor and you find out about it via the ceiling below. Re-sealing is an afternoon; a ceiling is not.
Bleed the radiators and check the boiler pressure at the start of the heating season. Cheap, and it catches a slow leak before it is a cold house in January.
Clear the drain gullies outside where they take water from a downpipe. A blocked one backs up against the wall.
Check the loft after heavy weather, once a year, with a torch. You are looking for daylight and for damp on the timbers. A slipped tile found now is a tile; found in a year it is a rafter.
Each is fifteen minutes to an hour. Between them they prevent nearly every expensive water-related repair, which is most of what goes wrong in a house.