This paper describes the development of a prototype systematic web-based decision aid for selecting low-carbon refurbishment solutions for multi-story residential buildings in high-density subtropical cities, with special reference to Hong Kong. 88 potential general sustainable refurbishment approaches are identified from a comprehensive literature review, later narrowed down to 39 considered to be relevant to Hong Kong's climatic condition and building characteristics, and then to 17 solutions for common areas and 14 for privately-occupied areas after interviewing industry experts and verification by questionnaire survey of the occupants. A set of methods is then established through literature review and energy simulation based on a case study, to calculate the CO2 emission reductions achievable. The system is then described, which includes a set of criteria for identifying individual applicable refurbishment solutions, methods of calculating CO2 emission reductions, parameter input/output and user interface design for a particular building. Finally, a validation process involving industry experts is followed to ascertain the system's practical usefulness and possible refinements for implementation.