The manual for my high-efficiency washer says I may need to reinforce the floor. What does this mean? New subfloor? How big a job is this? What can I expect to pay?
Generally speaking, if your floor is supporting its current use (a laundry area) and isn't otherwise rotting or in disrepair structurally, there is no reason why it needs to be reinforced. However, high-efficiency washers can vibrate. That is because the drum rotates much faster than a regular washing machine during the spin cycle.
It's very important that both the floor and the washer are level to minimize the vibration. To remedy it further, you can get an anti-vibration pad to place under the washer. There is a company called KE Shake-Away that makes a very good one.