“My house and my neighbor’s house share a common sewer line; that is, my sewer line crosses over into their property and then connects with their sewer line. The combined line connects with the main sewer running underneath the city street. The neighbor has started doing major renovations and has broken the sewer lines as part of their foundation work. He claims that I must now re-route (at my expense) my sewer line so that it flows directly to the main sewer. Both buildings are very old and there are no recorded easements. Who is responsible for repairing the sewer line?”
While a definite answer does depend on who owned the sewer lines and what legal rights each property owner has (historical questions), I would guess that your neighbor–or his contractor–is responsible, since they broke the existing line.
However, there may be some facts that I’m not aware of which affect this situation. In particular, if your neighbor thinks you aren’t entitled to reinstate the prior situation (shared sewer line), perhaps there is a reason for that. I’d want to know what that is before I jumped the gun and took a strong position on the argument.
I’d start by asking the neighbor why he thinks there needs to be two lines instead of the prior one, and see where that takes you.