I found this cutaway of the ground floor of the Simpsons' house:
But, I don't remember the top right room in any episode. Does it really exist, or is it made for the cutaway?
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And here's another appearance, from Three Men and a Comic Book (Season 2, Episode 21):
The fact that it is rarely seen was parodied in White Christmas Blues (Season 25, Episode 8):
Marge: Well, there's a rec room off the kitchen but sometimes it's there and sometimes it isn't. Our house is very odd that way.