The current climate and environmental policy efforts require comprehensive planning regarding the upgrade of the energy supply and infrastructures in cities. Planning comprises e.g. the determination of locations for new power generating facilities like photovoltaic, geothermal and decentralized combined heat and power stations, the widespread introduction of e-mobility solutions and hence the grid development as well as large-scale energetic building refurbishments. A holistic approach integrating extensive complex information is essential for the strategic planning of the different measures. In order to establish interoperability and data exchange between the different planners, stakeholders, and tools, an open information standard is required. To answer this need, an international group of urban energy simulation developers, geo- information scientists and users from 11 European organizations is developing an Application Domain Extension (ADE) Energy for the OGC open standard CityGML. This paper presents the collaborative development of this new open urban information model, including its genesis, objectives, structure and next planned steps.