[eixample]

This point may go without saying, but the lack of density is not simply about single-family vs. multi-family but rather the size of the single-family lots. Philly, for example, is very heavily oriented towards single-family, but so much of it is row homes with 1000 sq foot lots or under that it is pretty dense for an American city (and would be so much denser if the vacant lots were filled up).