The creepy old mansion is actually the Allison home on Colonial St on the Universal backlot. It's located next door to the home used as The Munsters (1964) house.
The house in this "Quincy M.E." episode was indeed next door to The Munsters house on Colonial St. in Universal Studios. It's also an exact duplicate of Anthony Perkins' Psycho" house above his Bates Motel.
The house that is the focal point of the episode is the same house that was used for the Munsters TV series.