The house with its seventeen steps that Dr. Watson called "221B Baker Street" was in fact officially known as "30 York Place", but York Place was a short street joining Baker Street and Upper Baker Street, later relabeled. (York Place ran from Paddington Street/Crawford Street north to Marylebone Road.) There were and are lots of other York this-n-thats on the London map, so "York Place" without specifying "Baker Street" would have been hopelessly ambiguous. The modern (post-1930) number is 111 Baker Street.
Holmes's house was definitively identified by one Dr. Gray Chandler Briggs, based on his chance discovery around 1930 of a building actually bearing the plaque "Camden House", which we are told in "The Adventure of the Empty House" stands directly across from Holmes's building.
(Doyle claimed this was a total coincidence, and said he had not been in Baker Street for at least thirty years -- but from the Holmesian point of view, the identification is far too satisfying to give up.)