I think I know what your problem is. See, with your current setup, the house scene is not existent in your scene tree. All scenes have a parent viewport node called "root" but they are still separate scenes wich is why your "world" scene can not find it in the tree.
In order to make it work you need to do something like this:
var house = load("res://Scenes/House.tscn") # load scene from file
var node = house.instance() #Create a new instance of the house
add_child(node) #Add the actual node to the scene tree
And it is also a pretty good idea to only load the scene ones by calling:
onready var house = preload("res://Scenes/House.tscn) #This loads the scene once at execution before the game is running
Then you can simply instance and add the node later in your "build" function.
I hope this helped you! :)