This part of the boundary line between Canada and the United
States was established in 1842 by the Treaty of Washington. The
houses in question are certainly not that old. I would guess they
are 50-100 years old. So the houses were built across the boundary
rather than the boundary being run through the houses. Nearby
monuments must have made it obvious they were building on the
boundary. This type of construction is now prohibited as you can
read in the article at
http://www.geocan.nrcan.gc.ca/ibc/ibcpg2.htm#permission
BJB