Unlock a settlement's workshop to gain control of it and influence it. Each has a requirement that must be met before it can be used.

Settlement	How to unlock
Abernathy Farm	Complete the "Returning the Favor" quest.
Boston Airport	Complete the "Shadow of Steel" quest.
Bunker Hill	Complete the "The Battle of Bunker Hill" quest.
Castle	Clear the area of hostiles.
Coastal Cottage	Clear the area of hostiles.
County Crossing	Complete the offered radiant quest.
Covenant	Complete the "Human Error" quest or defeat the current residents.
Croup Manor	Clear the area of hostiles.
Egret Tours Marina	Placate or defeat Phyllis Daily.
Finch Farm	Complete the "Out of the Fire" quest.
Graygarden	Complete the "Troubled Waters" quest.
Greentop Nursery	Complete the offered radiant quest.
Hangman's Alley	Clear the area of hostiles.
Jamaica Plain	Clear the area of hostiles.
Kingsport Lighthouse	Clear the area of hostiles.
Murkwater Construction Site	Clear the area of hostiles.
Nordhagen Beach	Complete the offered radiant quest.
Oberland Station	Complete the offered radiant quest.
Outpost Zimonja	Clear the area of hostiles.
Red Rocket Truck Stop	Unknown.
Sanctuary Hills	Unknown.
The Slog	Complete the offered radiant quest.
Somerville Place	Complete the offered radiant quest.
Spectacle Island	Clear the area of hostiles.
Starlight Drive-in	Clear the area of hostiles.
Sunshine Tidings Co-op	Clear the area of hostiles.
Taffington Boathouse	Clear the area of hostiles.
Tenpines Bluff	Complete the offered radiant quest.
Warwick Homestead	Complete the offered radiant quest.