However this three storey house in Madre de Deus, Brazil at about five feet wide inside, is perhaps pushing the envelope a bit. It is all decked out inside with one foot square tiles so you can measure it out, which was kind of the designer. (the source website says that it is a meter wide but we are going by the tiles) The owner is planning a fourth floor addition "open, for leisure, with barbecue and everything".
More at ::ah boon
We have seen houses like this in Tokyo, but commenter Jean Naimard points out that one does not have to go so far afield-"Such a house exists in MontrÃ©al; it has been there for more than 70 years and has been featured in the famous novel "Bonheur d'occasion" ("The tin flute") by Gabrielle Roy."