Habitat for Humanity East Bay uses a lot of green building techniques and materials, proving that affordable housing can be built green, too. I've incorporated green things into our addition and remodel, and continue to learn more as I go along.
Some of it is absolutely, completely 100% free. Proper placement of windows and roof overhangs, for example, costs nothing. Put them mostly on the south side (in the northern hemisphere) and arrange them so you get direct sun in the winter and not in the summer, and you've got free solar heating that didn't cost you any more than putting the windows somewhere else. It's also green -- there's no question of where the materials came from, what went into making them, how long they'll last, etc., compared with putting the windows somewhere else (which windows you buy does bring up those questions, of course.)
But it's harder to figure out the 'greenness' of a lot of other stuff. Take flooring. There are a lot of green options these days: bamboo, stained concrete, natural linoleum, tile, cork and more. But they're not all equally green. Bamboo is a very renewable resource, as it grows to maturity in only 5 to 8 years. It's also very durable, as hard as red oak and lasts a long time, and it's beautiful. But the bamboo used in flooring is grown in China. Even ignoring socio-political questions of labor practices, it means the bamboo has to be shipped halfway around the world before it gets to your house... not the most efficient use of fuel. (Note: yep, I installed bamboo flooring in the bedroom in the addition. Based on what I know now, I'm not sure if I'd do it again unless there were a bamboo source closer to the U.S.) Cork flooring isn't nearly as durable, and comes from halfway around the world the other direction, from Portugal.
Or stained concrete. Concrete is somewhat recyclable, extremely durable, and it can be beautiful. With a high fly ash content, it's using what would otherwise be a waste product. It also makes for a wonderful heat sink for storing passive solar energy and releasing it at night. But it's not a very kid-friendly flooring surface. And concrete takes a lot of energy to produce and transport. Of course, you can't build a modern house that's up to building codes in many areas without some concrete, but it does bear thinking about just how much you use.
Or natural linoleum. Not to be confused with vinyl flooring (which took over in the 1950's, and was frequently infused with asbestos, and being made of PVC it outgasses nasty stuff), it's made from natural products. It can be dyed into a rainbow of colors and patterns. It's flexible and water resistant. But it only has a life expectancy of 30 to 40 years, which while better than vinyl at 10 to 20 years, is still a long ways short of hardwood flooring that can last 100 years.
That brings us around to tile. It's durable, beautiful, waterproof and doesn't necessarily use eco-unfriendly materials. It even works nicely with radiant heat and can supplement concrete for storing passive solar energy. But it's not particularly kid-friendly, and without radiant heat can feel cold in winter. And it's very heavy compared with bamboo, linoleum or wood, so if it gets shipped any distance, it's not a very efficient use of fuel, either.
So what's the right answer? I think "it depends". I'm still not sure what we'll put in the kitchen, but what I've learned since I started this project means I'm going to put a lot more thought into it.