The original earthship concept, as poet said, was basically to provide highly energy efficient housing using recycled and local materials. The original one looked pretty hobbity, but that's not at all necessary. It just happened to have been built (apparently) by some kind of acid head hippy who liked that look.
The basic concept was rammed earth and old tires as a structural material. There's no technical reason at all why you couldn't build like that in an urban setting, the major problem is getting it through the planning process I suspect. The closer you can get it to local vernacular architecture the better probably, although where the local vernacular architecture is Barrett Homes that's going to be a bit tricky. In places where thick-walled, small windowed cottages are found, it shouldn't be too difficult to make it look 'right' however.
