Smart cities represent a paradigm shift in urban development, leveraging advanced technologies to enhance efficiency and sustainability. However, implementing City Information Modeling (CIM) faces significant challenges across technical, data-related, organisational, security, and privacy domains, hindering interoperability and effectiveness. This study proposes a Unified City Information Modeling (UCIM) framework as a comprehensive solution to standardise and harmonise data streams and operational processes within smart city ecosystems. Integrating Building Information Modeling (BIM), Internet of Things (IoT), Geographic Information Systems (GIS), and Semantic Interoperability Framework (SIF), UCIM bridges gaps between city management systems, facilitating seamless communication and collaboration across sectors such as transportation, healthcare, energy, and public services. The framework will enhance data exchange and system integration, support informed decision-making and resource optimisation, and pave the way for resilient, efficient, and sustainable urban development. This research underscores UCIM's potential to transform urban management practices and recommends further exploration through practical case studies to validate its application in diverse urban settings.