This chapter applies J. L. Austin’s speech act theory to design function. Austin’s theory, originally developed for linguistic utterances, distinguishes between locutionary content, illocutionary force, and perlocutionary outcomes. While this framework has been applied to artworks, its potential in philosophy of technology, engineering, and design remains largely unexplored. This work serves as a proof of concept, extending Austin’s tripartite model to analyse design function. By drawing on Austin’s original theory and its later applications in art, the chapter develops a speech act account of design function and examines how individual or conventional uses of design artefacts can diverge from their intended functions. The discussion concludes with a summary of how this framework can be used to analyse value change in design and engineering contexts.