You know those eco-calculator things? They always work better in really dense neighborhoods where transit is prevalent, no one needs a car, and you can pretty much walk everywhere. If you live in multi-family housing, you will necessarily have less space, use fewer materials; and rely on transit for day to day life. I can’t buy a “green” residence 10 miles out in the country, that is 5,000 square feet and uses solar panels. Every thing going into that residence requires wheeled transport. Now, a high rise on top of a grocery store with a bus stop outside… that makes sense to me.