If the code belongs to all developers in a group and everyone can change everything, it should be possible for them to do so. Keep the design as simple as possible. XP works a lot with the Keep It Short and Simple (KISS) principle. In other words, for a system to be easy to change, the design should be as simple as possible. This is easier said than done, however.

Traditional development methods have learned to think ahead and to always think about functionality in a design that might have to be realized in the future, but these methods are based on the assumption that costs for changes increase exponentially. This is why XP is always trying to choose the simplest design to enable a functionality that must now be realized. Ideally, any future expansions can be implemented with XP without the usual extra costs. It also turns out that, all too often in the realization of a well thought-out design, it does not reflect current requirements. This may happen either because certain details have been overlooked during analysis and design, or because requirements have been adjusted. With XP, design does not come first, but instead follows the code.