Was before. I dont think it is anymore. Maybe a quest or event only?

yep, I have the impression that it was possible in the earlier versions ...
Because after a couple of turns (I'm still in early game) and all my provinces are already 100/100 built. So can't improve/progress anymore ...
wtf ?

What do you mean 100% built? What nationnare u playing? How many provinces?
It takes me ages to build all as elves

I mean a "5" province already have 5 buildings. My 2 provinces already have 2 building etc. No more free slots in my provinces. None.
Of course at begining of the game I didn't have these buildings. I built them.
I'm human / Mardoba - the peninsula south of Boruvian Empire. The date is 9 january 1393 - and all my provinces are fully built.

For example: this province of Chevalese is a "4" province and all my slots are already built., I can't anymore improve the province.

https://steamhost.cn/steamcommunity_com/profiles/76561198044865469/screenshot/948458733327013345

You have to plan your building/country around the available slots.

Note: there is 1 workaround, if you can enslave prisoners. Slavery will temporarily give a random province an economic boost -- it essentially raises the economic level by 1. This will have the side effect of creating a new building slot. If you build a building within the period of this "bonus" economy, the buildling will survive once the slavery effect wears off.

Not every country can enslave prisoners, however, and there are negative diplomatic effects.

Not if you enslave the entire empire and became the dominant super power of mega orcs on the continent there isnt!

There are buildings in some provinces for some factions (e.g Aqueduct) that increase the province level and number of building slots by 1. Other than that and the slavery thing you can't increase province level.

When you could the AI just spent all its time upgrading all its provinces to level 10 and pretty much never went to war. And the bigger the kingdom (so more provinces it had) the longer this took. So the bigger the kingdom the less warlike it was.

Once you've built all the buildings you have the only way to expand your economy is to invade other kingdoms and take their provinces over.