1. A user interface extension method for a componentized application environment, the method comprising: specifying in markup a set of user interface controls; further specifying in markup an extension to a user interface control in the set; substituting the markup for the user interface control with the markup for the extension; and, providing the markup for the extension and the set of user interface controls absent the substituted user interface control to a set of corresponding control factories for generation into user interface control instances defining a user interface for the componentized application environment.