It's even more complicated in Philadelphia. Most of the old row houses do share a wall, which leads to a quirk if you buy and tear down one house in a row: you

It's even more complicated in Philadelphia. Most of the old row houses do share a wall, which leads to a quirk if you buy and tear down one house in a row: you can't reattach to the existing wall. Your new house just lost several inches of width, for having to build a new exterior wall, even if it buts up right against the existing wall. (Source: a realtor friend in Philadelphia)