First of all, make the most of natural light; if this is not possible, install LED lights. Also, plan for good thermal insulation in walls and floors. You can assess whether there is anything that can be recovered and reinstalled. A good option is to install water-saving taps, ECO flush toilets, and a shower instead of a bathtub. Another idea is to opt for materials with a reduced environmental impact or recycled materials that are both durable and of high quality, as well as antibacterial and easy to maintain.