Maintenance is recognised as the most difficult and expansive activity of the software development process. Numerous techniques and processes have been proposed to ease the maintenance of software. In particular, several authors published design defects formalising "bad" solutions to recurring design problems (e.g., anti-patterns, code smells). We propose a language and a framework to express design defects synthetically and to generate detection algorithms automatically. We show that this language is sufficient to describe some design defects and to generate detection algorithms, which have a good precision. We validate the generated algorithms on several programs.