Life Cycle Assessment (LCA) is a widely known methodology for “cradle to grave” investigation of the environmental impacts of products and technological lifecycles; however, this methodology has not been yet broadly used as an eco-design tool among the practitioners of the building sector. We applied LCA on three conventional Italian buildings – a detached residential house, a multi-family and a multi-story office building . Our analysis includes all the life stages, from the production of the construction materials, to their transportation, assembling, lighting, appliances, cooling- and heating-usages during the operating phase, to the end of life of all the materials and components. We found that the operation phase has the greatest contribution to the total impact (from 77% of that of the detached house, up to 85% of the office building), whereas the impact of the construction phase ranges from about 14% (office building) to 21% (detached house). We carried further analyses to evaluate the influence of various optimizations of the buildings, e.g., more efficient envelopes and facilities, on the entire life cycle of the three buildings. In addition, we propose a methodological approach, which can contribute to the acceptance of LCA as a tool in the eco-friendly design of buildings, especially those buildings whose impact during the construction phase needs to be carefully checked, such as Nearly Zero Energy Buildings.