Overhead view of 221B Baker St, the fictional abode of Sherlock Holmes. An annotated version is available. The address didn't exist when Doyle wrote the Holmes stories but after the extension of Baker St, a building close to where that address would be started to get a lot of mail addressed to Holmes.
Almost immediately, the building society started receiving correspondence to Sherlock Holmes from all over the world, in such volumes that it appointed a permanent "secretary to Sherlock Holmes" to deal with it. A bronze plaque on the front of Abbey House carries a picture of Holmes and Conan Doyle's narrative detailing Holmes and Watson moving in at 221B.