This article is considered accurate for the current version of the game.
Overextension is game mechanic modeling the issues caused by rapid expansion.
Each non-core province, with the exception of same-culture colonies, cause overextension equal to four times province base tax. For example, a province with a base tax of 6 will add 24% overextension regardless of how big the state is. Note that overextension has no cap; it can potentially go far beyond 100%.
Overextension negatively affects the following:
|Penalty at 100% overextension||x% OE|
|-100% Foreign trade power||-x%|
|+50% Stability cost modifier||+0.5x%|
|+50% Mercenary cost||+0.5x%|
|-1 Papal influence||-0.01x|
|-2 Diplomatic reputation||-0.02x|
|+5 Global revolt risk||+0.05x|
In addition, the AI will view overextended countries more negatively, and will be more likely to either declare war itself, or form a coalition against that country. Overextension will only decrease the relations with nearby nations, so while an overextended France would suffer strained relations with Spain and Germany, Russia on the other hand would not care as much.
Above 100% overextension, various nasty events will trigger, doing things such as lowering stability or hurting trade. The base frequency is one bad event per year, with the time decreasing with higher overextension.
Overextension can be reduced by coring provinces, which take time and administrative points, or by selling non-cored provinces to vassals or other nations.