221C Baker Street is a basement flat owned by the landlady Mrs. Hudson. She has struggled to rent it out, mainly due to its mold and dampness.
221C was built on the same property as Mrs. Hudson's flat and 221B Baker Street. Mrs. Hudson bought the property and has tried to rent it out. However all these attempts were unsuccessful and the basement remained unused, mainly due to the mold on the walls.
When the consulting detective Sherlock Holmes first came to look at 221B, he also had a brief look at 221C.
Jim Moriarty used 221C Baker Street as the location for a part of his Great Game. After picking the lock, Carl Powers' shoes were planted in the room. Sherlock was later sent a picture of the room, causing him, John Watson and DI Greg Lestrade to go investigate.