Abstract: Embodiments of the present invention address deficiencies of the art in respect to extending the user interface of an application, and provide a method, system and computer program product for extending the user interface of an application in a componentized application environment. In one embodiment of the invention, a user interface extension method for a componentized application environment can include specifying in markup a set of user interface controls and further specifying in markup an extension to a user interface control in the set. The markup for the user interface control can be substituted with the markup for the extension. Thereafter, the markup for the extension and the set of user interface controls can be provided to a set of corresponding control factories, absent the substituted user interface control, for generation into user interface control instances defining a user interface for the componentized application environment.