Designing a software architecture is a highly complex task and associated with a high degree of uncertainty. There are a variety of reusable and established solutions, but they differ in their impact on the system's functionality and quality. The architect has to consider different aspects like stakeholders' requirements as well as numerous constraints coming, among others, from the technical environment and organization. The context of software evolution sheds a different light on constraints. The existing system with its structure based on previous decisions is a limiting factor constraining the ongoing development. However, current approaches do not sufficiently consider constraints induced by an existing system until now. To assist the architect in taking the right design decisions efficiently, tool support for the recommendation of solutions and structured documentation of the design decisions are indispensable. In our paper, we propose a decision process focusing on the consideration of constraints in evolving systems. Furthermore, we introduce our tool Decision Buddy and show how it contributes to the application of our constraint-based decision process.