Building in Washington, DC where the counter-espionage organization known as CONTROL leads their fight against KAOS. [Episode #1: "Mr. Big".] In addition to office facilities CONTROL also had state-of-the-art laboratories in their headquarters [first seen, Episode #2: "Diplomat's Daughter"] - until the lab is moved to a secret location backstage at the Follies Theatre [first seen, Episode #72: "Classification: Dead"]. Even though the base is supposed to be top-secret, KAOS and members of the public know the address to the facility. [e.g. Episode #48: "Cutback at CONTROL" and Episode #106: "To Sire, With Love, Part 1"]. [Note: the building with the sign clearly reading "Hall of Justice" which Max goes into when he comes to work in the opening credit sequence of the third and fourth seasons is actually located in Los Angeles.]
The exact location of CONTROL Headquarters is unclear. The address is variously given as 123 Main Street [Episode #48: "Cutback at CONTROL"] and W Street, corner of 34th [Episode #65: "Maxwell Smart, Private Eye"], and the outside of the building itself [as seen in the opening credits] appears to change through the years. It is possible that CONTROL actually moves locations from time to time but there is never any reference to this. More likely, these are multiple access points to the same underground facility.
Once inside, an agent must walk down a flight of steps, emerging through a pair of fake elevator doors into a long corridor protected by four sets of security doors. At the end of the corridor is a phone booth. Stepping inside, the agent inserts a coin into the phone and dials a secret number which activates a trap door dropping him in front of the door to an outer office beyond which is the office of the Chief.
Even further underground is the office of the Chief's superior [Episode #105: "Absorb the Greek"], presumably accessed via the elevator in the corridor near the phone booth [Seen only once in Episode #117: "The Treasure of C. Errol Madre"].
After CONTROL is decommissioned its files are archived in another building at 90427 43rd Street and the organization’s now-abandoned headquarters falls into disarray. 15 years after CONTROL is shut down Larabee still remains stationed in the collapsing building due to being confused by a memo from President Richard Nixon ordering him to stay at his post. [TV Movie: "Get Smart, Again!"].
At that time CONTROL is reactivated and by 1995 access is via a car wash, and with an entirely different internal layout that suggests it is now in a new location [ Get Smart (1995) ].