The variety of technologies and standards in the domain of service-based systems makes it complex to build architectures which fit specific project contexts. A reference architecture accompanied by guidelines for deriving context-specific architectures for service-based systems can ease this problem. The NEXOF-RA project is defining a reference architecture for service-based systems that serves as a construction kit to derive architectures for a particular project context. Experience in developing the reference architecture over the last two years has shown that the service-oriented context results in different and sometimes contradicting demands for the reference architecture. Therefore, the development of a single and integrated reference architecture is not feasible. Instead, for constructing the reference architecture, the project has chosen a pattern-based approach that allows the consideration of different types and demands of service-based systems. Thus it can deal with contradicting demands of different types of service-based systems and is extensible to include new future trends of service-based systems. This paper will present the structure of the pattern-based reference architecture and explain how it addresses the needs of a reference architecture for service-based systems.