Energy retrofitting can enhance indoor environmental quality, reduce energy consumption, and reduce greenhouse gas emissions in aging buildings. However, in New Zealand, energy efficiency mandates primarily focus on new residential and commercial constructions, leaving a gap in standards or guidelines for retrofitting existing school buildings. This study addresses this by assessing the energy-saving potential of a school in Christchurch under various retrofit scenarios. The research begins by creating a baseline energy model using DesignBuilder. The assessment follows a four-step process: first, calculating the energy use intensity (EUI) for heating and/or cooling according to the existing building; second, determining the EUI based on the minimum requirements outlined in The Designing Quality Learning Spaces; third, calculating the EUI aligned with the H1 Energy Efficiency Building Code, which targets new structural buildings; and finally, assessing a retrofit scenario following the EnerPHit standard. Operational carbon reduction analysis is also conducted. Results show traditional retrofits achieve significant energy savings, while EnerPHit standards offer substantial reductions in heating and cooling EUI. Improving the building envelope significantly reduces operational carbon emissions. This study offers practical insights for developing effective retrofit plans, comparing different standards, and achieving desired outcomes in energy efficiency and environmental impact.