The transition to a decarbonized energy system requires the adoption of climate policy in sectors such as buildings, industry, and transport. This climate policy integration is subject to political processes, but there is a lack of empirical investigations on how these political processes take place and which drivers shape integrative policy change. We address these gaps by conducting an empirical analysis of the 2024 recast of the Energy Performance of Buildings Directive (EPBD) of the European Union (EU), a part of the “Fit for 55” program aimed at integrating climate change mitigation into building policy. Specifically, we ask (1) how did levels of climate policy integration evolve in the policy process and (2) what were the drivers of changes in these levels? Based on a qualitative case study research design, we analyzed six interview transcripts, thirteen policy documents, and 59 newspaper articles. We find that the EPBD represents a significant increase in integration of climate policy with regards to policy objectives and policy instruments, but only a minor increase of integration in governance capacities. Levels of integration differed between policy venues and changed during the policy process, because of issue salience and political leadership. We suggest further research to investigate the role of interlinkages of these drivers for integration processes and derive as a policy implication that policy integration levels need integrative capacities at all stages of the policy process.