I did exactly that and closed it back up months ago! I will be adding the pedestals next to raise the washer and dryer, building in a cart garage for the pictured cart also. It ill have a counter top for folding and a few other odds and ends and then the painter will come in and paint it all.Since its all open now why don't you put in a washing machine box that has the drain and water valves. Looks nice too.
Guy Gray makes a good box.