html:'< div id = "id_area" style="width:100% height:100%" > < /div > '
However there are certain rules in that approach.First one is z-index. If you want to use navigation like elements, expanding menu may result in a div element whose level is lower than any other extjs based panel. To cope with this problem, you may want to use main panel's html property instead of using sub panels.
Second problem is about events. If you want to assign a method to an event, you need to implement that method outside of the Extjs onReady function since onReady will create its own scope.
There is also one more thing about html property. It can be cleared using div's innerHTML property, when you add an extjs panel into your main panel, main panel's html content will be ignored and your inserted panel will cover all of your main panel.