This paper reflects on our experiences during the last four years regarding the development of a web & mobile application for sustainable environment. After five development cycles our software application has become robust, flexible, and expandable platform. Inspired from this and the needs identified during this process we report our research towards identifying novel architectural approaches that nourish open data and emerging web standards for developing open societal applications. The efforts resulted in an open architecture approach that relies on a set of key characteristics - flexibility, evolvability, customizability, and extensibility. Stressing these characteristics refers to the identified needs of using open standards and reducing development time. The results show that an open architecture approach provides the key ingredients in terms of system integration and data interoperability as well as the ability of the software to grow in terms of new services, devices, and subsystems attached to it, primarily due to the use of open source components and open data standards.