Buildings are responsible for 39% of global CO2 emissions. It is estimated that 80% of current buildings will remain in operation by 2050, a significant proportion lacking energy efficiency. Consequently, retrofitting plays a pivotal role in current and future efforts to counteract global warming. Retrofitting involves modifying a building’s components, systems, or structure post-construction to enhance its efficiency, thereby improving occupant comfort and overall performance. This study focuses on retrofitting the energy performance of an office building situated in San Donato Milanese, Lombardia, Italy. The research commences by analyzing the energy consumption of the existing building. The goal was to keep the energy retrofitting interventions as straightforward as possible, taking into account the financial viability. Implemented measures include increasing envelope insulation, incorporating shading panels, and integrating renewable energy sources. Architectural modifications are also considered in the design to enhance the quality of office interiors. These design concepts are evaluated in terms of their impact on energy performance. The results of the detailed energy and daylight analysis for pre- and post-retrofit scenarios are presented. Finally, the effectiveness of energy retrofit scenarios in reducing the building’s global warming potential, and achieving low-carbon design objectives is quantified and discussed.