Extensibility is a system design principle where the current implementation takes into consideration future growth. It is a systemic measure of the ability to extend a system and the level of effort required to implement the extension. Extensions can be through the addition of new functionality or through modification of existing functionality. An overriding theme is this is done without impacting (changing) the current system functions.

Although usually applied to software, it can be applied to anything. Houses can be built with future extensions in mind. The Bloor Street Viaduct was built to accommodate a future subway line.