-
There's a sewer pipe there and we share a manhole and connection that runs under fence (and would have run under new party wall). Took us months to get an agreement with him about what to do, including him demolishing his sort-of-temporary extension. Then we found out the pipe went another way, then did a camera survey, then found out the survey was shit, then dug exploratory holes and found out it was different again, then dug further and realised we'd need to get Thames Water approval... It's been a whole thing. Added lots of time, stress and money to what was going to be a relatively straightforward thing.
I like the trench extension behind your neighbour's lines.