16 Building Slots - Skymods
Read more about 16 Building Slots at Buildings, Economy, Overhaul on Skymods.
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Each mecha has six slots for specialized parts, and they grant various stat boosts. Some parts increase melee damage, some increase health, some increase armor, and so forth.
How do you unlock more building slots? : r/hoi4
Basically, you increase your building slots by adding yourself the spirit (which is doing the job) from the decisions tab.