In response to increasingly stringent European Union directives and national regulations on the energy performance of buildings, reliable and interpretable energy assessment methods are essential, particularly for public utility buildings due to their large floor areas and significant contribution to national energy demand. This study compares regulatory calculation results obtained for an administrative public utility building using two frameworks: the energy performance certificate (EPC) procedure and the energy audit methodology. The objective is to quantify methodological discrepancies and to indicate which model assumptions most strongly affect ventilation–related losses and the resulting energy indicators. For the investigated building, the ventilation heat loss coefficient determined within the audit methodology was 37% higher than that obtained under the EPC procedure, which translated into an over 31% higher useful energy demand for heating and ventilation. The audit–based EU and EK indicators were higher than those calculated within the EPC framework, while the EP indicator remained at a comparable level, indicating that differences in methodological assumptions can substantially alter the distribution of useful and final energy components without necessarily producing a proportional change in primary energy. The reported differences should be interpreted as consequences of distinct regulatory algorithms and input treatment (including ventilation and infiltration air flow rates and system efficiency assumptions) rather than as evidence of divergence from actual operational energy use, as measurement–based validation was not performed. The findings support a more cautions interpretation of EPC and audit outputs and underline the need for broader case samples and operational data to generalise results.