Hearts of Iron 4 (HOI4) Update 1.12.3 Patch Notes - UpdateCrazy
Both: Add 2 Building Slots and Build Rocket Site at level 2 in random state with less than 2 levels of Rocket Site and available Building Slots Add Secret ...
72 Building Slots - Road to 56 - Skymods
Read more about 72 Building Slots - Road to 56 at Gameplay on Skymods.
Unique buildings - Stellaris Wiki
A bonus building slot at the start can kickstart your early game, but you have tons of ways to get more building slots by the late game.