House Purchase - Build Over / Public

JamesP2020

Hi all,

I am in the process of buying a house and close to exchange and have an issue for which I am having some trouble finding an answer. Maybe we are overthinking this.
The current owner built an extension in 2000. No building regulation certificates. I understand it's a long time ago and I can get an indemnity insurance for that. Had a survey done and not worried about the structural integrity.

My issue is that the extension went over a sewer pipe which services the neighbours property. My understanding **was** at the time it was built, circa 2000, it would have been a private sewer and therefore didn't need a build over agreement from Thames Water. In 2011 this changed and now it would be classed as a public sewer.

Judging by the layout (see pic) this looks like a slam dunk private sewer until 2011 type scenario. It only serves one neighbour’s property and does not connect to main sewer on the property boundary. 

However, I am getting conflicting information having spoken to a few different people in that there is chance the pipe could have been owned by Thames Water pre 2011 and therefore a build over agreement would have been required by Thames Water when the extension was done in 2000. The house is pre 1930s house – not sure if that makes a difference. In all my research it seems odd that Thames Water would own this particular pipe but it’s a question mark nonetheless.

I have spoken to the solicitors and the water/drainage search company and they cannot tell me when the sewer was adopted. If it was 2011 then no problem but if pre-2000 then we have a problem.

I know we can get another indemnity insurance or retrospective agreement.

I’m planning to eventually remove the extension and replace it but my concern is when I contact Thames Water for a build over agreement they may state the original structure was not allowed in the first place and I am not allowed to build a new extension on top of the pipes. This makes the indemnity insurance worthless. Not sure they would do this but it’s a risk and my biggest question/concern.

Retrospective agreement seems like a nuclear option for a hypothetical and I very much doubt the seller will agree.

It seems the only way to know when it was adopted would be to contact Thames Water which I don’t want to do (yet) as this will alert them and invalidate any insurance we may decide to take.

Does anyone have any ideas or have come across a similar scenario? How likely is it the pipes were owned by Thames Water pre 2011? Likelihood of a future extension being blocked by Thames Water? Do I go for the insurance/ negotiate asking price? Or do I just for the retrospective agreement?

Thanks