In the past few years, we have witnessed an increase in software functionality as an attempt to meet most users` needs. This approach brings about serious usability challenges, due to an increase in application complexity as well. In order to try and meet users` needs, without overloading the application with functionality that is rarely used, there is a tendency to allow end users to configure or program applications, by means of mechanisms that support the so-called end user programming. However, many existing techniques fail to attain acceptable thresholds of usefulness and usability. This work addresses some of the challenges posed by extensible applications. We follow an approach that drops some walls between interface and extension. This approach brings some extension mechanisms to the interface, and readily accessible to end- users, namely extensions based on the semantic-pragmatic resources of metaphors and metonymies. These mechanisms were chosen due to the acknowledgment of the Cognitive Sciences of their critical role in our reasoning processes, especially when we try to describe or understand complex or abstract concepts (Lakoff e Johnson, 1980; Lakoff, 1987; Lakoff, 1993; Ortony, 1993). We describe an extensible application model that makes use of a knowledge base in which we represent the domain and application elements that may be extended, as well as the necessary classifications for calculating the possible extensions. Our model takes into account the communicative aspects of computer applications, and follows Semiotic Engineering (de Souza, 1993) principles to guarantee the consistency between the original application and the extended one. For that purpose, our model entails the representation of rules that constrain interface amendments, so that extended functionality is adequately reflected at the resulting interface.