When I was looking at model houses about 10 years ago, one of the houses we went to had something I’d never seen before, a playhouse under the stairs, complete with it’s own light switch. Big enough for kids to go in and out but not big enough for out of shape adults, it’s been a hit with visiting kids. Here’s a picture with the lights on:
Here’s the picture of the inside:
The opening is 13.5″ inches wide, fyi. I highly recommend the cardboard bricks as well for the room, as they can be used to block the opening, which kids really seem to like.