I feel like I read an article on this topic every month or so, but the repetition is okay. The greenest building is the one that already exists and is reused. Or, as Mark Alan Hewitt puts it in Common Edge: Why Reusing Buildings Should Be the Next Big Thing.
Existing buildings vastly outnumber new ones, and buildings tend to last far longer than the thirty years used for depreciation. Renovation and reuse have more of an effect on the environment than new construction does.