Game concept
Duchy Building
The game also links it as Duchy Buildings.
From the game's encyclopedia
Duchy Buildings are powerful Buildings that can only be constructed in the De Jure Capital of a Duchy. Only one type can be constructed in each Duchy.
Duchy Buildings are only active if the owner also holds the associated Duchy Title.
Many Duchy Buildings have modifiers that affect all Holdings in the Duchy, making them powerful tools for specializing your Domain.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Duchy Building is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Duchy Buildings are powerful Buildings that can only be constructed in the De Jure Capital of a Duchy." It is part of Building. The game also links it as Duchy Buildings. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.