Dr. Gregory House’s apartment number in House M.D. was 221B.
The address is a deliberate reference to 221B Baker Street, the London home of Sherlock Holmes. House was heavily modeled on Holmes, and the series repeatedly used parallels between the fictional detective and its medical diagnostician. House’s address therefore works as an in-joke for viewers familiar with the literary source.
House’s apartment is shown as a private space where he plays music, uses drugs, argues with friends, and thinks through medical problems. The address also reinforces the idea that House investigates symptoms much as Holmes investigates clues.
The apartment number is not the same as the address of Princeton-Plainsboro Teaching Hospital, the fictional institution where House works. It is also distinct from 221B Baker Street itself: House’s apartment number borrows the number, but the show places the character in New Jersey rather than London.