The A350 has staircases IIRC ?
The only true double-deckers are the 747 and the A380. Some other wide-bodies have been configured with a galley below the passenger deck, so flight crew have stairs but not passengers.
Maybe there's a variant with lavatories below deck? If so, it's not common.
The A350-900ULR is used by Singapore Airlines on the Singapore - New York flight that clocks 18 hours and a half. To be able to fly that far it needs to be light on payload: few passengers, no cargo. So there's some empty space in there...