Grimmauld Place

From UnknowableWiki

Grimmauld Place is a small square in London with dingy and badly-kept houses (OP4). The Muggles who live in Grimmauld Place have long since gotten used to the numbering mistake that causes the thirteenth house to be after the eleventh (DH12).

When one has access to the secret that hides number twelve, home of the Blacks and at one point headquarter of the Order of the Phoenix, is revealed, pushing the other two to the side without any of the occupants noticing (OP4).