Who Is Liable?

[susanxx]

A neighbour has applied for planning permission for an extension to the rear of their property. Their house is the second in line of 6 houses served by the main sewer. According to the plans it would appear that the rear will be built over this main pipe. Should there be any future issue as in the sewer pipe breaks and subsequently affects the other 4 properties, would he and his insurance be liable to repair blockages/damage incurred by the other homes or would the other properties have to look to their insurance to repair the issue?

[davebro]

They will need a Build Over Agreement with the water company who should ensure that the work is done correctly.
Once a second or subsequent property joins the sewer system the pipework becomes the responsibility of the water company & they should sort out any issues.

[The Builder]

May I just add to Dave's answer.
Water Companies have the right to refuse this. Normally, they do their best to accommodate the request, but often it is only if a simple diversion of the sewer is unfeasible.

Being second in the line, the sewer serving that house is the responsibility of The Water Company.
Don't worry. The Company will look after all of this.