The Apartment Building

The apartment building in which the stories take place, located at 2311 N. Los Robles Avenue in Pasadena, California. It's located about a block from Pasadena City Hall, which is visible through Sheldon and Leonard's window. The building has at least five floors and sixteen apartments The elevator has been broken for at least two years, but finally explained how it got broken during The Staircase Implementation. Lastly, it contains the laundry room in the basement.

Layout of the Building

 * Basement – laundry room
 * First floor – mailboxes, glass front doors, broken elevator.
 * Stairway
 * Second floor - Bike in front of broken elevator, water jug by 2A. Mrs. Grossinger lives in 2A
 * Third floor - There are often flowers on plant stand by 3A, including poinsettias before Christmas. Mrs. Vartabedian lives on the third floor.
 * Fourth floor
 * 4A (Sheldon and Leonard)
 * 4B (Penny)
 * Plus a probable third apartment
 * Fifth floor
 * Alicia lives in 5A.
 * Roof – there is a 3 foot gap to the next building

Where is the building located?
Here's a wonderful post from the forums by Ottonomy on the subject:

I believe I have pinpointed the building from which those views from the apartment window would have been possible. It's a commercial building owned by Western Asset, its official address 385 E Colorado Blvd (go to the post to see the photo, as viewed from the Colorado & Los Robles Ave side). It's a five story building, and from the back side, one would be able to see the dome of City Hall beyond a tile-roofed apartment building (circled in blue in the photo) in view on the right, about half way between. From this vantage point, looking northwest, some foothills should be in view in the background. The parking lot of Western Asset exits onto Union St, from which the first logical turn to head to CalTech would be southbound onto Euclid Ave.

One of Pasadena's few red light cameras is installed on the corner of Union and Marengo Ave, where Sheldon was caught running the light on the way to the emergency room with Penny. This would be the logical path to Huntington Memorial Hospital from that location.

Los Robles Ave does indeed stop just shy of the fictitious 2311 North address, in the middle of the 2000 block, which is about 20 blocks north of Colorado Blvd.