Owned - Own Diamond City
In the vanilla version of Fallout 4 you can purchase a single dwelling in Diamond City. With this mod, you can take over the entire thing.
Owned - Own Diamond City allows you to purchase most of the properties located in Diamond City so that you can use them however you see fit. These are acquired these through special Property Ledgers.
Sadly (or not, depending how you view the residents there), you'll need to commit a few murders before the people will fork over their land.
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Once you own a property, the mod grants you access to workshops and more. It's an interesting mod and one that's worth a try if Diamond City is feeling stale.