What is this dashed bounding box left where there is no geometry? I have created a house using a plane. Then I decided to redo the house. 
I kept the old house to see the dimensions, and after re-doing the house, I deleted the old house.
However, Blender still shows the box of the old house.



Why is that so and how can I delete this artifact? There are no vertices in the old house anymore, so I am not sure why Blender still shows this box.
Those dashed lines normally represent the object's texture space when Texture Space is enabled in Properties > Object > Display:



You might have accidentally pressed &#8679; ShiftT at some point, which is used for translating the texture space. As a side effect it enables texture space display as in the screenshot above.

If you disable texture space display, then they should go away.