This article is considered accurate for the current version (1.12) of the game.
Each non-core province, with the exception of same-culture colonies, causes overextension equal to province development. For example, a province with a development of 24 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||Penalty per point of overextension|
|-100% Foreign trade power||-1.0%|
|+50% Stability cost modifier||+0.5%|
|+50% Mercenary cost||+0.5%|
|-25% Better relations over time||-0.25%|
|-2 Diplomatic reputation||-0.02|
|+5 National unrest||+0.05|
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.
At over 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 to let them core it. Note that they will not accept unless they have a core on the province, or it has the same culture group and religion to the country.
Overextension is not increased by seizing provinces in wars started under the Colonial Conquest Casus Belli.
Administrative efficiency is a country wide bonus that is unlocked at administrative technology level 23 and increases at 27, up to a total of 50%. Administrative efficiency directly reduces core creation and diploannexation costs, also the impact of province development on overextension and warscore cost, allowing for much larger territories to be conquered at once.
The scaling of core time from country size has been removed - all nations now core provinces at the same speed regardless of size (but it is still affected by factors such as culture, religion, having a claim and so on).