Ken of This House explains how a single house can exist in both the United States and Canada at the same time by harking back to the early 20th Century when borders were not as definitive as they are now.
Just north of the US state of Maine sits Campobello Island, a scenic landscape offering panoramic views of the ocean. On maps, it appears to lie just outside the US, in Canada, but upon closer inspection, you will find that there is a house resting on shared soil. So the question arises, how did this come to be? And how could a house belong to two different countries?
The house, which is the main feature of Roosevelt Campobello Island, belonged to the Roosevelt family and served as a summer home for future President Franklin D. Roosevelt After Franklin’s death of the home was sold to billionaire Armand Hammer. When Eleanor Roosevelt died, Hammer then deeded it as public space in a most confusing manner.
Rather than founding a non-profit to run the house as a museum of Americana in Canada and then seeding it with plenty of money to sustain itself, he chose to create what was possibly the most confusing real-estate deal in the joint histories of the two nations by gifting it both the US and Canada. …the two governments formed a treaty. It was decided that the estate would become an international park which would be jointly owned, controlled, and maintained by both governments.
via Nag on the Lake