I think that happens to a lot of people when they're reading. I know it happens with characters, too. I have very strong images of characters and then their hair colour gets mentioned and I have to rearrange my whole portrait of them.

I've been looking at a lot of London housing lately, for stories, and there are a lot of places that have 'void' areas. They are mostly houses converted to flats, where new walls were built and areas become closed off. I suppose 221 could work if there were some void areas in it. Otherwise it is a non-rectangular flat in a rectangular building.