A customization environment uses a customization system that models customizations to a base computer system. The customization system receives developer inputs that customize the design time behavior of a form control by modeling it with an extension model. Based on the developer inputs, the customization system generates an extension expression that expresses the design time behavior of the form control. The extension expression can express the design time behavior through the addition of attributes representing the design time behavior or through code authored in an extensibility framework.