Energy Performance Certificates (EPCs) have been used as a simplified method to provide an energy rating of buildings alongside recommendations on how to improve it using Reduced Data Standard Assessment Procedure (RdSAP) for existing buildings. However, this methodology often fails to reflect the actual energy performance of traditionally constructed buildings, which are categorically “hard-to-treat” due to their fabric characteristics, reliance on passive ventilation and heritage constraints. Drawing on our in-depth study of responsible and safe retrofit measures for TLDs in Southeast England, this paper uses case study (CS) approach, to identify major limitations of current EPC assessments, including unrealistic energy efficiency ratings and energy savings projections, and insufficient consideration for heritage sector. The study compares EPC-predicted performance with results from Dynamic Energy Simulation (DES), calibrated with real monitored data from four CSs. Findings reveal that EPCs significantly underestimate the potential energy savings achievable through sensitive whole house retrofit strategies. This research contributes to the debate around EPC reform and highlights the need for more accurate assessment methods, particularly for traditional heritage properties, as the UK moves toward ambitious energy efficiency targets for 2050.