> You can safely put a washer and dryer on the upper floor of a house if you add a proper floor drain

European here, we've always been placing washer/dryer stacks in our bathrooms without needing floor drains. We have machines with a system called Aquastop [1] that stop the water flow if the machine detects a leak inside the system.

[1] https://de.wikipedia.org/wiki/Aquastop