The integration of multi-temporal SAR interferometry with Building Information Modelling (BIM) provides new opportunities for object-level deformation monitoring in urban environments. The BIMSAR framework demonstrated the feasibility of linking Persistent Scatterer Interferometry (PSI) results with IFC-based BIM models to derive building-specific deformation indicators. Multi-frequency InSAR data from Sentinel-1, TerraSAR-X, and PALSAR-2 are processed using a PSI-based strategy to derive millimetre-scale deformation velocities and time series. Building on this methodological foundation, this study reports the transition of BIM–SAR fusion toward an API-enabled Digital Twin service, including the delivery of a structured deformation database and the presentation of an interoperable API design for scalable building deformation monitoring. Deformation measurements are semantically associated with BIM objects and, where applicable, building components. The fused information is stored in a structured database that has been implemented and delivered as part of the service development, and is exposed through a RESTful API to provide standardized access to deformation indicators and quality metadata. A pilot implementation in the city of Ahlen, Germany, demonstrates the end-to-end workflow from MTInSAR processing to BIM-linked data storage and API-based digital twin visualization. The results show that deformation time series can be systematically integrated into BIM environments and served through interoperable interfaces, supporting reproducible and scalable geospatial digital twin services for building deformation monitoring.