This is an interesting question! As the previous Educator has stated, there was never a 221B in Baker Street until relatively recently, when the Sherlock Holmes Museum took up residence at a house specially renumbered 221B for their purposes. However, Baker Street is, of course, a real place, and just as in Sherlock Holmes's time, it is situated in Westminster, London. Even the postal code is actually a Victorian invention: London's NW post code dates back to 1858, although the corresponding postal codes for the rest of the United Kingdom did not come into use until the mid- to late twentieth century.

The purpose of the postal codes is to indicate where a place is, in relation to the central post office, which was in the region of St Paul's cathedral. So, Baker Street lay to the immediate northwest of this location. However, the postal code would not have been so specific in the time of Sherlock Holmes. The addition of the 1, for example, dates back to World War I, when subdistricts were added to each postal code in order to make it easier for the sorting office to deal with post.

221B Baker Street is a fictional address. There is a Baker St. in Westminster City in London, but there has never been a 221B.

Sir Arthur Conan Doyle was in fact outside of London for much of the time when he wrote his stories and novels on the Sherlock Holmes character. He was a Scottish writer and author and is believed to have written the stories using a detailed map of London.