The stairs exist in the fictional apartment building at 2311 Los Robles Avenue that serpentines around the broken elevator for a distance of five floors where Sheldon, Leonard and Penny reside.
For the original second pilot, which introduced the character of Penny, the original sets from the first pilot were modified to add an elevator next to the steps. By making the elevator non-functional, many scenes used the steps for a place for discussions by arriving and departing characters, random meetings and the subjects of visitors' complaints. The downstairs laundry room is also used for random meetings. The same set of stairs is used for all three sets of stairs by changing the hallway decorations and apartment numbers. A separate set is used for the building lobby.
Significant appearances in episodes
In "The Justice League Recombination", Sheldon runs down the stairs dressed as the Flash in his imagination.
In "The Zarnecki Incursion", Penny and Priya share a long, awkward silence as they walk up the stairs after an encounter at the lobby.
In "The Infestation Hypothesis", Amy and Penny go running down the stairs screaming after seeing something alive in her chair that she had retrieved off the curb.
In "The Big Bran Hypothesis", Leonard and Sheldon drag Penny's new IKEA entertainment center up the stairs.
In "The 43 Peculiarity", a jealous Leonard follows Penny's classmate down the stairs telling him that her boyfriend is a jealous gang member. From the lobby, Leonard sees that Penny witnessed his deception and storms up the stairs getting mad.
In "The Cooper-Nowitzki Theorem", Penny follows Ramona Nowitzki up the stairs trying to verify that she is visiting Sheldon Cooper; tall skinny, looks like a praying mantis.
In "The Nerdvana Annihilation", the guys are bringing their time machine upstairs blocking Penny's ability to go downstairs. She ends up going up, crossing the gap between The Building and the one next door and ending up having lunch with an Armenian family.
In "The Codpiece Topology", Sheldon retreats to the stairwell to play video games while Leonard has his dates with Leslie Winkle.
In "The Bus Pants Utilization", Sheldon takes his theremin and plays it in the stairwell after being thrown off Leonard's app writing team.
In "The Hofstadter Insufficiency", the pizza delivery guy wants more of a tip for climbing the stairs. Penny offers him thirty plus cents and a promise not to tell his boss about the marijuana aroma around him.
In "The Proton Resurgence", Professor Proton doesn't make it up all the way up the stairs without Sheldon, Leonard and Penny coming down to the third floor to help him.
In "The Cooper Extraction", Amy learns that Sheldon uses her picture for his screen saver and sees a physical demonstration on how he cares for her. When Sheldon returns she is staring at him all the way up the stairs, asking if he missed her and asking how he liked holding his new-born nephew, therefore might like to have a baby with her.