If building a green home from the ground up, local codes and the ground itself are good places to start. Finding a design and materials that match what you hope to achieve in sustainable living is a solid starting point, but where you build has a lot to do with how you build.
Constructing foundations typically involves some type of poured concrete, with a sub-level plan that includes drainage points for directing water away from the foundation. Concrete isn't the greenest of materials, though many use it in combination with green products and techniques. Long-term energy efficiency often is a part of the planning, and the foundation will determine how the home will settle over time, and how many nooks, crannies and creepy crawlies -- not to mention how much water -- will get into the house through the ground.
Land and soil conditions vary by region and climate, so building with site-specific considerations such as sandy soil, rocky, and uneven ground or flooding, and wetland cycles in mind is important. Consulting national and local building codes for load-bearing guidelines and restrictions is essential. Some wood-frame and Earth-rammed homes are brilliant in design, but they may not pass code. An adobe house or Earth-filled foundation could crumble in an earthquake, so it may be flagged in areas prone to earthquakes before the ink on the plans even dries.