[johnsjmc]

I just went through almost exactly the same thing about a year ago. I bought a 110 yr old house in the core area. The sewer was shared with the neighbour and the majority of the line including the street connection was on the neighbours property. We knew when buying the house ,the sewer was shared but had been assured it was in good working order .HaHa.
At closing it was (but not for long) . Turned out it was the second flooded basement in 5 years and my insurance company would not pay for a third unless we replaced our side.
We had a very big rainstorm which flooded both basements because it exceeded the capacity of the single line.(roof runoff is combined into the sanitary in the old neighbourhoods ,now needs a separate storm drain).
I had a liner company estimate close to 25k to do both sides. We ended up using the old method of a back hoe and trench. Even that came to about 10-12k. The city required a new connection at a cost of $5k from the road centre to the property line and my contractor did the rest for another $5k. I managed to keep costs down a little because my hoe operator coordinated with the city and did the actual digging at both ends as a subcontractor to the city sewer dept.
The neighbour contributed to restoring the driveway and kept the old sewer with a spot repair . He now has a single old sewer and I have a brand new sewer complete with a backflow preventer valve. The city came up with a 3k grant to me for installing a backflow valve and a sump pump for rainwater . Total cost to us both was about $12-15k with me paying most, but I also got a partial grant of 3k . Talk to your city sewer dept and get a traditional quote . The trenchless liner jobs are hugely more expensive than a dig and only save if infrastructure like driveways and fences etc are going to cost more to replace than digging. I also found out the city used to subsidize new sewer connections to eliminate shared connections when found BUT that program was ended when they started giving grants for backflow valves. Consider several opinions and talk to the city about any subsidy or maybe even who their subcontractors are . The city might also allow repair to existing without requiring complete replacement , but a new line is better and I prefer all new not a linered old drain. Good luck.
My wife is a retired realtor and asks If you weren,t told about the shared sewer do you have any legal recourse with either the seller or the agents or both?