Continuous changing requirements of software are the result of continuously changing reality. This reality can be considered as the context of the software. Agile development methods allow quick adaptations to changing requirements. Initially, agile development methods were focused on the development of the application core only. Recently, process models were discussed that integrate HCI aspects. This paper will discuss ideas to integrate user evaluations into the development process. User interfaces are structured as UI-pattern instances. Tool support is provided that allows the specification of pattern instances as XAML specifications. Additionally, the tool allows the replacement of one pattern instance by another one. In this way, different versions of the same user interface can be generated rapidly without much effort. These different versions can be evaluated with the help of users. Based on these usability tests final decisions for the software design can be made. New requirements can be captured additionally. This will be based on feedback of the users.