Energy performance certificates (EPCs) have gained increasing attention in recent years in policies and research. However, little attention has been paid to the consistency of EPC input data over time and the ability to assess EPC quality by comparing it with prior or subsequent EPCs. This study analysed the EPC input data from 52,000 Danish single-family houses with two or more EPC reports. In total, about 105,000 EPC reports were investigated. Statistical analysis of the building geometry and thermal building envelope revealed significant changes. The results show that the building envelope area varies nearly independently of the reported floor area. Furthermore, the U-values of the opaque envelope increased by 45% of all cases, and the one for the transparent envelope parts increased in 16.5% of the cases. Additionally, 20 EPC report sets were manually analysed, and five illustrative examples were provided to use information that was not readily available on a large scale. This analysis showed that changing assumptions regarding building constructions and a rapid information loss between consecutive EPCs, even for newly built buildings, were the primary sources of change. The analysis also indicated that changing input values could offset renovation measures. The results demonstrate that inconsistencies across multiple EPCs for the same buildings are typical in Danish EPCs. The study suggests that professionals issuing new EPCs should have access to input information from existing EPCs and be obligated to revalidate existing assumptions and document deviating decisions.