Embodiments of the present invention provide techniques for customizing aspects of a metadata-driven software application. In particular, embodiments of the present invention provide (1) a self-contained metadata engine for generating customized metadata documents from base metadata documents and customizations; (2) a customization syntax for defining customizations; (3) a customization creation/update component for creating and updating customizations; (4) a customization restriction mechanism for restricting the creation of new customizations by specific users or groups of users; and (5) memory and caching optimizations for optimizing the storage and lookup of customized metadata documents.