Not actually my house, it was an AirBnB outside Seattle, but it's mysterious and I'm hoping someone here might know the function.
Inside the main closet in the master bedroom is a weird giant staircase leading to a square hole that looks into the area outside the bedroom. Here's the other side:
The inside of that cupboard beneath is just a small empty space, much smaller than the staircase, tiled at the bottom and possibly sized for an electric fireplace or something.
My friends and I could not figure out what it was supposed to be, and it's driving us a bit crazy.
The outlet is definitely a newer feature, you can see the plaster has been redone but not painted, so it's probably a red-herring. From the photo, the window looks a bit bodged in too, so it may not be an original feature.
This is a bit of wild speculation, but with the external cubby hole, the large platform-like steps and a small window, could it be that someone tried to build a small sauna room? The heater would go in the cubby as they're not allowed/don't want a traditional type setup, and the room would have been lined with the traditional sort of cladding, with vents to let the steam in. You could sit on the steps as normal, and have a washroom right outside. If it was later decommisioned, the cladding removed and redecorated and the heater taken out, you'd end up with something that looked like that.