In frigid environments, antiquated office structures are typically marked by elevated energy usage owing to the constraints of their building shell. This research centres on Xi’an, a city famous for its frigid winters and oppressive summers. This study uses DesignBuilder V6.1 to develop energy-consumption models for standard buildings, assess the energy-saving potential of external wall insulation, window selection, and roof insulation retrofits, and analyse the return on investment for each initiative. The implementation of an expanded polystyrene external thermal insulation system can lead to a reduction in total annual energy consumption by roughly 25–30%; employing low-emissivity double-glazing alongside polyvinyl chloride window frames can decrease heating and cooling energy usage by about 12–15%; notable energy-saving advantages are noted when the thickness of Extruded Polystyrene insulation boards on the roof is below 80 mm, whereas the incremental energy-saving benefits lessen when the thickness surpasses 80 mm. Considering the energy efficiency and cost-effectiveness of various measures, this research recommends a strategy that maximises energy-saving advantages per investment unit: an Expanded Polystyrene external wall insulation system, Low-Emissivity double-glazed windows, Polyvinyl Chloride window frames, and Extruded Polystyrene roof insulation boards ranging from 20 to 80 mm in thickness. This plan achieves an optimal equilibrium between energy efficiency and affordability, making it highly suitable for extensive implementation. This research serves as a benchmark for the initial selection of energy-efficient renovation strategies and the development of policies for analogous older office structures in Xi’an and areas with similar climatic and architectural characteristics.