The apartment building, or simply the building, refers to the apartment building at 2311 North Los Robles Avenue, Pasadena, California. When viewed from the satellite photographs in "The Countdown Reflection" episode it appears to be located at 215 South Madison Avenue. Leonard, Sheldon, and Penny occupy apartments on the fourth floor. Adjacent to a lamp store, it is located about a block from Pasadena City Hall, which is visible through Sheldon and Leonard's window. The building is also located near a Chevron station on Colorado Boulevard, being that Leonard makes the excuse that he uses its bathroom when he and Penny began seeing each other for the second time in season 5.
The apartment building has at least five floors and sixteen apartments (based on the number of mailboxes). The Elevator has been broken for several years, after it was damaged by one of Leonard's experiments involving rocket fuel, as explained in 'The Staircase Implementation' causing everyone to take The Stairs. The building also contains a laundry room in the basement.
During the initial production of The Big Bang Theory , Chuck Lorre stated that the set department had built him a stairwell with three walls, and he didn't know of what to put on the third wall being that it was the most visible. One of his producers suggested that Chuck Lorre put in a broken elevator, which would add to the reason why the characters had to use the stairs.
The only separate sets built for the apartment building (albeit the apartments) are the lobby (ground floor), the laundry room, and the roof. The stairwell is the same set for all shots, but accessories and details are changed inside the hallway between each shot filmed to create the illusion that the actors are ascending or descending each floor.
The laundry room is located here. The main power switch is also in the basement.
The first floor contains the lobby, which is the entrance to the apartment building. It also contains the mailboxes of the residents who live in the building. This also contains the elevator doors that say out of order, or have the caution tape over them. It's also suggested that the first floor has unseen apartments on it, being that this is the floor which housed tenants from whom Leonard bought the couch.
- Mrs. Grossinger lives here, along with her hairless Mexican dog which Sheldon dislikes. The second floor also has a bicycle in front of the broken elevator door, and a water jug by 2A.
- Mrs. Gunderson lives here at least in "The Lunar Excitation" (S3E23). There are often flowers on a plant stand by 3A, including poinsettias before Christmas. Judging by the wall behind Mrs. Gunderson, the floor plan of 3A is different from that of Sheldon and Leonard's apartment upstairs.
- Mrs. Vartabedian also lives on the third floor. Seen as Mrs. Gunderson lives in apartment 3A then she must live in apartment 3B.
- Sheldon (already lived here in "The Staircase Implementation" in 2003)
- Leonard (joined Sheldon in "The Staircase Implementation" in 2003)
- Sebastian (left here in "The Staircase Implementation" in 2003)
- Penny (joined Leonard in "The Cohabitation Experimentation" in 2016)
- Rah (briefly in "The Roommate Transmogrification" in 2011; currently joined Leonard and Penny in "The Escape Hatch Identification" in 2017)
- Penny (Since The Pilot in 2007, left here in "The Cohabitation Experimentation" in 2016)
- Louie/Louise (already lived here in "The Staircase Implementation" in 2003, and left before The Pilot in 2007)
- Sheldon (moved here in "The Cohabitation Experimentation" in 2016)
- Amy (moved here in "The Cohabitation Experimentation" in 2016)
- Alicia (since the "The Dead Hooker Juxtaposition" in 2009).
There is a 3 foot gap to the next building, access to the roof and enough room to set up astronomical experiments. This is also the location that Howard and Bernadette were married
In case you haven't noticed, the veining in the marble on the elevator walls, and in the light sconces are exactly the same on each floor. (They use the same set for each floor during stair-climbing or descending scenes.
The earliest version of the apartment building is seen in the episode "The Staircase Implementation".
The first resident of the building that we see in "The Staircase Implementation" is Louie/Louise when Leonard accidentally knocks on the door to his 4B apartment instead of Sheldon's, 4A. We also catch a glimpse of Sheldon's former roommate named "Sebastian" (Steven Yeun) exiting the then functioning elevator, and he warns Leonard about Sheldon. We also know that there is an apartment opening up on the first floor being that Leonard buys the sofa from them for a hundred dollars. As of the fifth season, Sheldon has been living at 2311 No. Los Robles for eight years.
Penny is moving into the building during the Pilot episode, and Alicia moves into an apartment on the fifth floor during Season 2.
In "The Staircase Implementation", we learn that the elevator was working at this time being that Leonard uses it to get to the fourth floor. We also learn that Leonard had miscalculated the formula for an experiment that he was doing with government grade rocket fuel that he was developing. Seeing that the cylinder was about to explode, Leonard attempted to take it out of the building, via the elevator. Sheldon knew that Leonard would not have time, so he pulled Leonard out of the elevator and closed it with the cylinder inside. The cylinder exploded and permanently disabled the elevator. Leonard considers himself being in Sheldon's debt not only for saving his life, but not telling the landlord or the government about what happened. This is the reason why he chooses to keep living with Sheldon.