Change map when player executes an action

Hello! I want to add a certain feature to my game. It is a horror game.
I want the player to go inside a house and pick up a flashlight. When the player picks up the flashlight I want to edit the landscape. Basically, make a new path appear when the player steps out the house. How could I do this seamlessly, with no major stutters from loading another slightly modified level? Thanks in advance!