Community Z Tools (CZT) is an integrated framework for the Z formal specification language. In this paper, we show how it is also designed to support extensions of Z, in a way that minimises the work required to build a new Z extension. The goals of the framework are to maximise extensibility and reuse, and minimise code duplication and maintenance effort. To achieve these goals, CZT uses a variety of different reuse mechanisms, including generation of Java code from a hierarchy of XML schemas, XML templates for shared code, and several design patterns for maximising reuse of Java code. The CZT framework is being used to implement several integrated formal methods, which add object-orientation, real-time features and process algebra extensions to Z. The effort required to implement such extensions of Z has been dramatically reduced by using the CZT framework.