@suufiee Well, you didn't actually create a "room", just a wall. Walls attach differently depending on how you draw them. If you draw them "towards" the house, they'll be different than if you draw them "away" from the house. They attach differently.
Try just drawing the walls away from the house. I mean start at one of the existing house walls and draw away. Don't try messing with the foundation until you've enclosed it in a square. If you've attached it properly to the house, the entire house will raise up and down.
The fact that just those walls went down and not the entire house means they weren't attached to the main house properly.