In response to the increasing functional and spatial demands in dense urban environments, retrofitting existing building structures has become an essential practice in modern construction. This research investigates the application of extra-long pre-cambered I-beams integrated with composite concrete slabs in a high-rise apartment garage retrofit project in Kuala Lumpur, Malaysia. The project required extending the original 10 m column grid to a 21 m column-free span, while maintaining the safety of 10 overlying floors. A systematic methodology was conducted as such: (1) a 65mm pre-cambering strategy was designed based on finite element simulation and verified through field calibration; (2) segmented beam fabrication and confined-space lifting procedures were developed to address site constraints; (3) shear connection and composite action between steel beams and concrete slabs were optimized according to BS5950, BS8110, and Eurocode 4; and (4) an IoT-based system was used for real-time quality assurance and safety monitoring. The structural performance analyzed was further experimentally verified through full-scale on-site water loading test and a 90-day long-term creep-shrinkage monitoring program. Findings showed that net deflection was reduced to 3.8 mm, below the 58.3 mm allowable limit, while subjected to 1.2 times the design live load, the maximum measured stress reached only 66% of the steel's yield strength, confirming the elastic structural behavior. The Pavilion Mall case study further demonstrated the scalability and effectiveness of the technology, yielding a 26.7% reduction in construction time, 12–15% cost savings, and significant improvements in load-bearing capacity (+30%), deflection control (−45%), and clearance height (+14.3%). Beyond immediate technical benefits, this study highlights pathways for standardization, industrialization, intelligent construction integration, and sustainable development, with potential to reduce carbon emissions by approximately 15% compared with conventional methods, which confirmed that pre-cambered steel–concrete composite systems provide a feasible, efficient, and sustainable solution for long-span retrofitting projects in confined urban spaces.