A building life cycle assessment tool was developed and linked with an energy simulation tool, allowing to take into account the strong interactions between the energy and the environmental performance of buildings. This life cycle assessement tool was then extended at the urban level to evaluate the environmental impacts of a district including many buildings, streets, public spaces and technical network. Building and neighbourhood design alternatives can thus be compared, providing assistance in the decision process. However, the reliability of LCA tool is essential to guide the decision maker towards sustainability. A methodology is presented here to investigate the most uncertain inputs. Parameters affecting the building energy performance and the choice of the electricity mix were identified as the most influencial factors which could alter the results’ robustness.