221B Baker Street is the London address of the fictional detective Sherlock Holmes, created by author Sir Arthur Conan Doyle. In the United Kingdom, postal addresses with a number followed by a letter may indicate a separate address within a larger, often residential building. Baker Street in the late 19th century was a high-class residential district, and Holmes's apartment would probably have been part of a Georgian terrace. The residence was introduced in the novel A Study in Scarlet (1887). At the time the Holmes stories were published, addresses in Baker Street did not go as high as 221.

Source: Wikipedia