Urban Green Building
"I happened to notice that very little, if not at all, mention of urban dwellings and how small urban homes are practically the greenest you can get when you factor in transportation. Green homes spread out in the country, unless you're living off the earth and have no use for a car, may counteract your carbon footprint savings if you have to drive on a continuous basis. A vast majority of Americans live in a metropolitan area, it would be nice if your information can include an aspect to the benefits of small homes in urban dwellings."
"I am curious about building an earth covered or underground home in the future. Can these houses be built on a small lot within a city? I think being close to your neighbors etc., is one way to help achieve sustainable living, however, the green homes I have seen always appear to be on a large parcel of land."
I think these folks are absolutely right about this. It is unfortunate that most of the natural building movement has been more of a rural activity...but there is no reason why it has to be. Virtually all of the principles of sustainable architecture that I outline at http://greenhomebuilding.com/sustainable_architecture.htm would equally apply in an urban setting.
In districts where housing goes above 2 or 3 stories, it is difficult to use some of the more natural techniques. One problem is that many of these methods of building result in rather thick walls, especially when the walls must go quite high, so that interior space is compromised by this. This is where some hybrid concepts might be useful, such as building with a steel framework to allow multiple stories, and then fill in the walls with less industrial materials, such as strawbales, cordwood, or earthbags.
As for going underground in a city, it certainly can be done. It would be a great way to create dwelling space and reserve most of the land above for gardening or parks, creating much needed green space in the city.
Also much of the movement towards "sharing facilities," such as co-housing, can be done in cities. This is another way to create both denser housing and reserve open space for parks and gardening.
I think that all proponents of green architecture need to put more creative thought into urban design!