Dr Gordon Finlayson, the author of Habermas, A very short introduction has an interesting project he is working on about the nature of immanent criticism. The project is entitled "What is immanent criticism/critique? Why should social criticism be immanent?" In his book he has the following comments concerning Habermas and immanent criticism:
". . . like Horkheimer and Adorno before him, Habermas employs the method of immanent criticism. One can also call it internal, as opposed to external criticism. The critical theorists think this approach derives from Hegel and Marx. In some respects it is closer to the Socratic mode of argumentation, which assumes the position of the interlocutor, for the sake of argument, without actually endorsing it, in order to point out its incoherence and untruth. Whatever its origins, the critical theorists aim to criticize an object - a conception of society or a work of philosophy - on its own terms, and not on the basis of values or standards that transcend it, in order to bring its untruth to light." (Haberams, AVS, p. 9, emphasis retained).
I believe Finlayson here confuses immanent criticism with internal criticism and considers them synonyms. In fact they are not synonyms. Immanent criticism can include both internal and external criticism and is opposed not to external criticism but to transcendent criticism. Habermas' approach is not an internal criticism it is rather an immanent criticism and does not exclude external criticism. This is crucial difference between Habermas' approach and Gadamer's approach (for example). Immanent criticism in Habermasian context means that the basis of critique must be inthe actual rational practices and is not to be derived from any other source which is located beyond these practices. However for Habermas these immanent practices provide us resources to go beyond them from "within" without appealing to any transcendent. Thus immanent criticism is very closed linked to Habermas' project of ‘transcendence from within.’