The construction sector is a major contributor to global energy consumption, with existing buildings representing a significant challenge in the transition toward energy-efficient and low-carbon solutions. Upgrading building envelopes is often constrained by site-specific challenges, the need for customized solutions, and budget limitations. In this context, prefabricated systems have emerged as promising solutions, offering advantages in terms of quality control, construction speed, and reduced waste and on-site disruptions. This contribution explores how off-site manufacturing (OSM) solutions, such as prefabricated panels, can enhance energy retrofitting efforts, comparing them with interventions carried out mostly on-site. It focuses on a pilot case study of an existing residential building, where two different renovation strategies were applied. One involves innovative, timber-based prefabricated panels, while the other adopts conventional retrofitting techniques—specifically, an external thermal insulation composite system (ETICS), a widely used solution, entirely realised on-site. The design approach and the details of the timber-based prefabricated panels are outlined, along with the entire intervention process—from panel production and on-site installation to the assessment of implementation challenges. The benefits and limitations of off-site manufacturing for renovation components are highlighted, comparing it with conventional renovation techniques and evaluating the scalability and replicability of such interventions. Key findings show that prefabricated systems can result in time and labor savings and improved quality control. However, challenges persist regarding higher initial costs and logistical complexity when compared to traditional methods.