Rowhouse lines

Looking out my back window I noticed something odd about one of the houses. And it is a weird little thing that I wondered about regarding my house, and other houses around. As these are townhouses, all connected and everything, is there like an easement or something that allows part of your house to be attached to your neighbors.
To better illustrate here’s the deal: Houses A has a closed in rear porch thing that leans (possibly due to age) into what looks to be House B’s side of the property line. What’s on House B’s side from House A are the gutters, roof overhang, and flashing. There is no adjoining structure like a porch on B’s side. Another example, my house, from the front it would appear that the fence and the paint line show where one house ends and the other begins. Nope. Discovered this when making a vent for the basement, the hole wound up on the other side of the fence in the neighbor’s yard. Similar problem in the back. Where the kitchen chimneys meet is a little less than a foot on the other side of the fence and the neighbor’s AC/heat pump thing sits right on the fence.
Now back in 1870-whatever when the landlord who owned my side of the block had these houses built, the exact line of where one house began and the other ended probably didn’t matter much. He owned the whole row of near cookie-cutter houses. But in time they got sold off bits and pieces to other landlords and until the late 20th century these units were almost always rental housing, so those with the responsibility to maintain and repair, didn’t have to live with the results.
The not-exactly cut and dry line of this side mine this side yours can create problems when it comes to fences, additions, weed trees, repairs, etc when the relationship you have with the owner of the neighboring house is not the best.