The (already canceled) show, Friends With Better Lives, has turned into my guiltiest of pleasures. Not only is the writing and casting as flat as can be, the sets are a laughable stereotype. The extreme conventionality of the living room set inspired me to dig deeper into the sitcom living room, and explain the reasons for some of the conventions.
A residential multi-cam sitcom (Friends, Seinfeld, New Girl, etc.) is focused around the living room because there are infinite plot lines that could take place either in, or passing through, the room.
To maximize the number of story options, you need to maximize the number of ways people can enter and interact with the space. The most versatile sets are the ones that incorporate the largest number of the following:
Front door, for surprise guests
Kitchen door, for pulling someone aside for gossip
Stairs, for bringing the party up to the bedroom
Guest bedroom, for an old relative to come out of and say something snarky
Window, for losing the family bird
Powder room, for finding a mysterious pregnancy test
Hall closet, for a place to hide if you have no clothes on
The key elements are the front door, kitchen and the stairs. Everything else is a bonus. You will notice that the stairs often land in the direction of the front door, so you have an intuative understanding of how the space functions.
That 70's Show was able to incorporate this formula into their show twice because the basement set was really a living room for teens:
But the most important part of the living room is the central couch. This is place where everyone can sit together for family meetings and to watch their favorite sitcoms. People sit facing forward staring through the camera and into a phantom television that will never be seen.