Does a dishwasher or washing machine have to be installed by a plumber?

The line falls where it does for everything else in plumbing: connecting to existing provisions with the manufacturer's fittings is maintenance, altering the provisions is plumbing work.

Generally fine to do yourself

• Screwing the supplied inlet hose onto an existing dishwasher tap.
• Placing the drain hose into an existing standpipe or connecting it to an existing dishwasher outlet.
• Levelling the appliance and plugging it into an existing outlet.

Licensed work

• Installing a new tap or drain point where there was none, which is what a new kitchen island or a relocated laundry needs.
• Cutting into or extending supply or waste pipework.
• Installing a new power outlet, which is electrical work.

The detail that causes the leaks

Most failed self-installations are not the connection itself but the drain hose. It needs a high loop or an air break so waste cannot siphon back into the appliance, and it needs to be secured so it cannot vibrate free. A hose that has worked loose inside a cabinet discharges into the cabinet, and cabinetry hides that until the floor is ruined.