To tackle the emergencies of climate change, Germany has committed to become climate neutral by 2045. The German Committee for Structural Concrete specified in a technical guideline a set of annual reduction targets by means of reduction classes for the greenhouse gas emissions of structures. The goal is to support practitioners to comply with mandatory emission reduction requirements. However, opportunities for the emissions mitigation through early design stages depend on appropriate choice of the load-bearing system, its members, geometry, and materials. Notwithstanding, decisions regarding minimal emissions, while guaranteeing structural safety and functionality, remain challenging in practice. This paper analyses how two reinforced-concrete slab systems (in-situ concrete and precast hollow core) perform in terms of reduction targets with the aim to give practitioners means to select a system that complies with such targets. To this, the effect of the span width, loading conditions and concrete classes with different cement mixtures is investigated. The results indicate that from 2030 onwards, adopting in-situ reinforced concrete slab systems, especially for large spans, might become challenging. Precast hollow core slabs seem to offer more design possibilities in terms of span width. Yet, the combination of CO2-efficient cements and concretes with optimised design approaches (e.g., prestressing) offers the largest potential for emission savings. This study is relevant to efficiently account for greenhouse gas emission objectives in early design stages. Furthermore, it appraises the possibility of transferring such targets to different slab systems (e.g., timber-concrete composite slabs) and, ultimately, other load-bearing systems (e.g., foundations or structural walls).