From Muppet Wiki

Gordon and Susan's apartment is upstairs in the brownstone at 123 Sesame Street. The apartment was used as a hangout spot in the early Sesame Street episodes, particularly the kitchen and dining room area.

In the first episode, Susan invites Gordon and Sally upstairs for some milk and cookies. They're joined by Bob, who hangs a picture on the wall, and Kermit the Frog, who watches Bob's magic trick.

The Sesame cast visit the apartment often in the early episodes, including episode 0008 and episode 0043.

Big Bird stayed there for two days while his nest was being rebuilt after a hurricane destroyed it. (Episodes 3976-3980.)