What is the impact of a basement on a building LCA and what role does the functional unit play?

The goal of reaching nearly zero emissions in the construction sector by 2045 is important and at the same time ambitious. Therefore, emissions from operation and structural design of a building play an important role. Life cycle assessment is an established method for determining the environmental impact of a building throughout its life cycle. The implementation of an LCA follows in general the standards ISO 14040 / 14044, and on building level the standard EN 15978. It requires, among other things, the definition of the goal, the system boundaries and the functional unit. For buildings with basements, there are certain discrepancies in the definitions of goals, system boundaries, and functional units. When the production, construction and maintenance phases of a building are included in the system boundaries of the LCA, the whole building is considered and is for example related to the functional unit gross external area or others as net external area or heated area. The set goal, system boundaries and selection of the functional unit has a direct influence to the LCA results. In this paper, the effects of a basement on the LCA results of the operational and structural design of a building are presented as a function of different functional units. Therefore, a comparative LCA on residential buildings in two different energy standards (level 40, level 55) and additionally for conventional and future-oriented construction were calculated.