More than merely software or an application, a Local Digital Twin (LDT) is a continuously growing means to maximise the value of existing and new datasets and models based upon a unified platform. Cities and organisations often face multiple digital initiatives focused on applications designed as integrated systems of data, business logic, and user interfaces. However, data and models can be of much more value if they are extended to be used for multiple purposes in an interoperable way. The strength of the LDT lies in the ability to aggregate existing and new technologies that enable the integration of data and models to deliver analytical and visual insights tailored to the needs of different users. The key to achieving this outcome is creating architecture based upon clear design principles and reusable, modular building blocks.