221B Baker Street

221B Baker Street was a property owned by Mrs Hudson which was home to Sherlock Holmes and John Watson in the late 19th century. (PROSE: All-Consuming Fire, Erasing Sherlock) Mildeo Twisknadine believed that both 221B and Sherlock Holmes were entirely fictional. (PROSE: The Taking of Planet 5)

History
Sherlock and Watson had lodgings in 221B by 1882. Gerard Cavendish and Thomas Corkle also rented rooms at this time. Gillian Petra became a maid at 221B to secretly observe Holmes and Watson. (PROSE: Erasing Sherlock)

In 1887, the Seventh Doctor, Bernice Summerfield, and Ace visited Holmes and Watson at 221B. (PROSE: All-Consuming Fire)

In 1893, the Eighth Doctor, Fitz Kreiner, and Anji Kapoor rented Sherlock Holmes' rooms while Holmes was "on an extended journey abroad". (PROSE: Camera Obscura)

Other references
The Museum of Things That Don't Exist contained correspondence sent to 221B Baker Street. (PROSE: The Taking of Planet 5)

The Dover Shores area of the City of the Saved contained a recreation of 221B Baker Street in its recreation of Victorian era London. For a time, this 221B was home to Broken Holmes. (PROSE: The Case of the Pipe Dream)

Behind the scenes

 * According to Stealing Series 9 Scripts (which, due to arguably breaking the fourth wall, is currently not considered a valid source by this Wiki), a "hideout volcano lair" below 221B Baker Street existed and was the place from which Steven Moffat wrote his Doctor Who and Sherlock scripts.