What is the origin of the idea that vampires cannot enter your home without being invited? Does it apply specifically to homes, or does it include any building? A walled garden? A vehicle?
The belief goes back at least to the 17th century. Leo Allatius recorded in De Graecorum hodie quorundam opinationibus (On the Beliefs of the Greeks, published in 1645) that a vrykolakas cannot harm the occupants of a house unless they answer the door:
For very often, inhabiting this body, he [the devil] comes forth from the grave, and going abroad through villages and other places where men dwell, more especially at night, he makes his way to what so ever house he will, and knocking upon the door he calls aloud by name in a hoarse voice one who dwells within. If such a one answers he is lost; for assuredly he will die the next day. But if he does not answer he is safe. Wherefore in this island of Chios all the inhabitants, if during the night they are called by anyone, never make reply the first time. For, if a man be called the second time it is not the vrykolakis who is summoning him but somebody else.
The origin of the belief is probably much, much older.