After the completion of the mission Boston After Dark given from Doctor Carrington and after the quest Butcher's Bill has been started, if the player character talks to P.A.M. it will say that, for the stability of the Railroad, it is vital to expand themselves with another settlement. It then proceeds to give the Sole Survivor the task of liberating a random settlement.
Once the settlement has been cleared, the Sole Survivor will be required to build sufficient defenses for the settlement. 10 defense is required to pass this step. Finally, when that is done, return to P.A.M. for the reward.
To start this quest, talk to P.A.M. inside the Railroad Headquarters anytime after completing the mission Boston After Dark. She will instruct the player character to construct a new settlement to expand the reach of the Railroad. P.A.M. states the settlement must be constructed and it will be a random settlement location. If having cleared all of the settlement locations it will immediately be a successful mission. Speak again to P.A.M. If one hasn't cleared all locations, travel to the random location and proceed to clear the area, then construct defenses around the settlement until the settlement's defense rating is at least 10. Barricades do not count as defenses until manned, and because there are no active settlers at this point, the player character will be required to construct turrets or traps to gain the necessary 10 defense. After this task is complete, head back to the Railroad headquarters and proceed to talk to P.A.M., to claim the reward.
Upon completion, a fast travel marker will appear beside the regular settlement marker, saying "Mercer Safehouse."
It is possible for the location chosen to be a settlement that the player has cleared out but hasn't interacted with the workshop at yet.
The location the Sole Survivor gets sent to is determined upon receiving the Butcher's Bill quest from Dr. Carrington. The quest automatically begins after completing the conversation with him at the end of Boston After Dark. To select a safehouse, create a save in the middle of this conversation, finish talking to him, then speak to P.A.M. who is now ready to give the Mercer Safehouse quest. Reload from the save point until the desired location is given.
Repeating this process might only give options like Starlight Drive-In, Outpost Zimonja, Hangman's Alley and Sunshine Tiding Co-op if they haven't been cleared as settlements; other settlement locations are provided once reaching certain levels if the aforementioned settlement locations haven't been cleared. It is recommended to save before turning in Boston After Dark and then follow the process of either clearing out these locations first or reaching higher levels until P.A.M. provides new locations.
The required player level for Coastal Cottage, Murkwater Construction Site and Jamaica Plain is level 40. The requirement of Spectacle Island is even higher.
The player character can still change the safehouse location even after the Butcher's Bill quest via console command. This is achieved by typing ClearQuestAliases RRR04 and SetQuestAliases RRR04 successively until the desired location is selected. The ID of the chosen safehouse location can be checked by entering ShowQuestAliases RRR04 - the "REF 'Workshop'" key will point to the refID of the workshop where the safehouse will be located. Note that once this quest is completed, these commands no longer have any effect, i.e. the safehouse cannot be moved.
If the safehouse location is moved by the aforementioned console commands, you will need to clear and build defenses for that location in order to complete the quest. Should the new location already meet the prerequisites, the quest will automatically complete once you fast travel there.
Changing the safehouse location via the console will not update the quest text for the location in your Pip-Boy.
The name "Mercer Safehouse" may be a reference to the prophet-like character Wilbur Mercer, from the novel Do Androids Dream of Electric Sheep.
The name "Mercer Safehouse" may also be a reference to General Hugh Mercer; a Revolutionary War general and physician that was also good friends with George Washington. Singer Johnny Mercer; singer of the song "Personality" played on Diamond City Radio is a direct descendant of Hugh Mercer.
The name "Mercer Safehouse" could also be a reference to Mercer Frey, a character from The Elder Scrolls V: Skyrim's Thieves Guild, with which the Railroad shares some similarities.
The name "Mercer Safehouse" also might be a reference to voice actor Matthew Mercer, who played the role of MacCready.