I think it makes sense that they didn't differentiate the chimney and all because Number 12 wasn't abandoned long enough for there to be any significant deterioration. It was left vacant for ten years, and yeah, that takes a toll on a building to a degree. The Order spent months cleaning the interior because those ten years DID make itself noticed with dust and dead animals under sofas. And it did take a bit of a toll on the outside too, from seeing (what I think is) the gutter is all bent up. But it wasn't a long enough time to screw up the chimney or roof. There's an article the Huffington Post did about a house that had been completely vacant for almost thirty years, and the roof looked pretty dang good given it wasn't kept up for years. If a building that's been abandoned for thirty years can have a semi-decent roof, why not a building that's only been left alone for ten?