The ever growing size and complexity of software systems is making it increasingly harder to build systems that both meet current and future requirements. During architecture design, a lot of important design decisions are taken. In this paper, we present an architecture design notation based on UML's activity diagrams. The notation allows for the specification of architecture fragments and supports composition of these fragments as well as superimposition of the fragments on each other. This notation allows us to make various compositions of architecture fragments (reflecting design decision alternatives) to adapt the architecture to new requirements. We have found that our notation is very suitable for modelling separate concerns at the architectural level.