Can you turn off the water mains to an apartment or unit?

In most cases yes, you just won’t be able to shut off water to the entire complex. If you need to shut off the water supply to a specific unit or apartment, you usually have to rely on individual shut-off valves located within the unit itself. Common places to find these valves include under sinks, behind toilets, or near water-using appliances such as washing machines or dishwashers.