How do you see this?

When two house drains join together it becomes a sewer.  Generally the Water/Sewer Utility companies are responsible for sewers, so clarification would be required from them on construction over a sewer.  Now, extensions to terraced houses over sewers are normally OK, but the foundations need to have a big fuck off lintal directly over the sewer, even if it’s way below the footings.  Which makes sense, you don’t want the weight of the extension on the sewer & where the sewer is it’s not virgin soil either.

As for the SVP & drains in the extension, it really depends on the layout, sewer & drain invert.  It maybe easier to connect to his own drain, as the sewer could be very deep. 
HTH