As global efforts intensify to mitigate climate change and reduce carbon emissions, energy-efficient retrofitting has emerged as a critical solution for enhancing the sustainability of existing buildings. This paper explores advanced retrofitting strategies aimed at minimizing energy consumption and carbon footprint while extending the operational lifespan of structures. Key methodologies include the integration of high-performance insulation materials, dynamic glazing, energy-efficient HVAC systems, and renewable energy sources such as solar photovoltaics and geothermal heating. The study also examines the role of smart building technologies, including IoT-based energy management systems and predictive analytics, in optimizing energy performance. A comparative analysis of conventional and modern retrofitting techniques highlights the economic and environmental benefits of upgrading aging infrastructure. Case studies demonstrate real-world applications, showcasing reductions in energy demand, improved thermal comfort, and enhanced indoor air quality. Furthermore, the research addresses challenges such as cost implications, regulatory constraints, and technological feasibility in large-scale implementation. The findings emphasize the need for an integrated approach, combining policy incentives, advanced materials, and digital technologies, to accelerate the transition toward low-carbon buildings. Ultimately, this paper provides a roadmap for stakeholders in the construction industry to adopt energy-efficient retrofitting strategies, contributing to global sustainability goals and the decarbonization of the built environment.