Yes, sorry, but 221B is in fact a house number and it does belong on the front door.
Until the fire department came through and standardized, my house number was 313A, and my neighbor to the south, 313B. In fact, despite being informed of the renumbering, the utility company still thinks I'm 313A, and my bills from them arrive reliably.
What, haven't you guys ever seen a duplex (semidetached house on your side of the Atlantic), a row house, or a townhouse subject to such a numbering scheme? And, my sister adds, she never met a city without it.
Neither she nor I grasp how Mr. Moffat and Mr. Gatiss could ever have mistaken 221B for an apartment number and therefore inflicted "221C" (as a basement apartment) upon us.
Therefore my site is "The Garret, 221B Baker Street."