Some are. Look for timbers from sustainable sources, the best insulation in walls, windows and doors and general quality that will ensure the building lasts. Modern base systems such as ground screws and pads can help avoid the movement of many tonnes of waste moved from site and new materials arriving. More locally sourced materials and recycling of waste materials will help reduce your carbon footprint too. https://www.pefc.co.uk/about-us/pefc-uk/