This research examines the technical feasibility of a Deep Energy Retrofit (DER) solution for multi-unit residential buildings (MURBs), using Habitations Dublin-Fortune as a case study. The building was selected for its suitability based on its condition, structural features, and socio-economic relevance within Montreal’s housing stock. The primary objective is to improve energy efficiency and aesthetics through a panelized retrofit system. The study explores key design elements, such as panel composition, structural considerations, and connection systems, ensuring the solution addresses thermal performance, air tightness, and building code compliance. Sustainable materials with low embodied carbon, non-combustibility, and lightweight properties were prioritized. A key challenge addressed is thermal bridging around balconies, which is mitigated by overcladding the building envelope and introducing new self-supported steel balconies. The research also examines the integration of energy-efficient HVAC systems, ensuring long-term sustainability and improved indoor environmental quality for residents. This pilot project demonstrates the potential of prefabricated DER solutions for retrofitting similar buildings in Montreal and beyond, providing valuable insights into practical implementation. The study offers a comprehensive framework for building retrofits, combining energy efficiency with improved living conditions for residents, highlighting how these innovative solutions can be adapted for a wide range of building typologies in urban and suburban contexts.