Approximately 40% of global greenhouse gas emissions result from the construction and operation of buildings. The continuous increase in these emissions underscores the urgent need for transformative action in the building sector to achieve climate protection goals. To support this transition, the research project “Development of a “Model System” for Assessing the Sustainability of Residential Buildings (new construction, renovation, and extension)” was initiated to promote sustainability in the residential construction sector and to facilitate knowledge transfer on a broader scale. Until now, sustainability assessment systems in Germany have primarily focused on new construction and the renovation of non-residential buildings, while residential buildings and their existing stock have been insufficiently considered. With the introduction of the “Qualitätssiegel Nachhaltiges Gebäude (QNG)” (Quality Seal for Sustainable Buildings) by the Federal Ministry for Building Affairs, an instrument has been introduced to promote sustainability in the residential sector by linking certification to public funding schemes. Against this backdrop, a model assessment system for was developed within the research project, covering new construction, renovation, and extensions of residential buildings. Based on an analysis of existing systems, certification criteria and indicators were developed in a pilot phase that incorporated practical insights from real case projects. Furthermore, an extensive commenting process, supported by advisory workshops, enabled the formulation of concrete recommendations for the further development of the QNG alongside the model system. The results provide a foundation for existing certification schemes and regulatory frameworks, highlighting the need to refine the system boundaries and promote the integration of sustainability principles in residential building assessment. The paper presents the outcomes of the project and outlines the future development of certification tools and regulations in Germany.