Component extensions must semantically conform to the components they extend to guarantee consistency of the extended system. Semantic conformance usually means preservation of observable properties while decreasing nondeterminism; in the presence of explicit invariants it also involves preservation of invariants by the extended and the extending components. Depending on the reuse technique employed for constructing extensions, the requirements that must be imposed on components to guarantee consistency vary. We concentrate on the issue of ensuring consistency of extensions with forwarding as the reuse technique, formulating requirements that allow consistent extension of components in the presence of explicit invariants. Also, we discuss additional problems arising with the use of inheritance and propose solutions to these problems.