Sometimes that can happen in England too... not necessarily London (I think so but I haven't evidence), but some cities, that large and old houses, very beautiful in themselves and once luxurious residences for the rich, get split up into flats and then become run-down and poorer areas. ;_; And as well as big old houses, often the Victorian terraced houses, which are pretty nice and quality houses (although not as large as those I think you are talking about) constitute the 'bad' areas of the city, whereas the horrible tacky matchboxes of the suburbs are the 'posh' areas.
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(not always, since there are some 'posh' areas of terraces, particularly where they are larger and better-maintained, but...)