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%.
These are the effects at 100% overextension:
|−100%||Trade power abroad|
|+50%||Stability cost modifier|
|−25%||Better relations over time|
|+0.50||Bureaucrats faction influence|
|+0.50||The guilds influence|
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:
- Rebel faction progress increase by 20% instead of 10%.
- 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 takes time and administrative power
- Selling non-cored provinces to subjects to let them core them (you may be able to annex them later)
- Selling provinces to other nations
- Releasing a vassal that has a core on it
Note that a country will only accept buying a province if they have a core on the province, or it has the same culture group and religion.
Overextension is not increased by seizing provinces in wars started under the Colonial Conquest Casus Belli.
Administrative efficiency is a country wide bonus that starts at 0% and increases by 20% at administrative technology level 17, 23 and 27, up to a total of 60%. Administrative efficiency directly reduces core creation and diplo-annexation costs. It also reduces the impact of province development on overextension and warscore cost, allowing for much larger territories to be conquered at once.