Which is greener; Building a home or Buying a home?

So I'm becoming more interested in being self sufficient and environmentally friendly. I would like to build a dream home out of straw bales and mud on a farm in the country with solar panels and a recycling water system. Then I started wondering if buying an existing house then using recycled building materials for refurbishing so the house can be more environmentally friendly. So, which is better at achieving my goal, building or buying?