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Discuss Potential drainage issue - next door extension in the Plumbing Forum | Plumbing Advice area at PlumbersForums.net

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We live in a terraced row (SW London) and our neighbours are currently in the process of finalising their extension. There is a potential drainage issue (their rainwater drainage) for which they have advised us of a course of action to resolve it. Before agreeing, we would like a bit of guidance on whether there are risks on our part, and have been trying to find it (without success) from many sources, so thought I would try here! For clarification, we would like to have a good relationship going forward with them, so do not want to be difficult for the sake of it, just to make sure agreeing to the work doesn't land us with issues!

Summary of the situation:
  • Next door (terraced house SW London) is extending their downstairs, and, in doing so, found issues with a rainwater drainage sewer (the clay pipe had cracks in it). They fixed this issue with the neighbour on the other side (which caused refooting work to be completed), and laid a new pipe coming up to our border under their new extension, on the assumption our house connected to it and it continued under our garden.
  • However, it doesn't - the pipe stops at the boundary, and our house (as verified by planning documents back to 2006) has both waste & rainwater all connected to the main sewer via a single connection on the other side of our garden to the boundary (about 6 foot away or so). I presume this pipe was here at some point, but it certainly hasn't been there for 15 years at least (as verified looking at council planning documents)
  • Next doors builder subsequently progressed with the building of the extension over the pipe (that is currently pushing rainwater from the terrace (unclear how many houses) onto the boundary. This extension is also over the main sewer, which runs parallel to the rainwater sewer. We assume that it is only rainwater flowing into the pipe, but recently have heard that there may be other waste going in - this is completely unclear at the moment
  • We have been told by our new neighbours and the builder that it is not now possible to have a connection between the rainwater sewer and the main sewer on their land. Their proposed approach (they are contacting Thames Water to confirm/get approval) is to route the rainwater sewer to our existing full connection to the main sewer, and join to that. This connection is where all our waste water connects to the mains. This work would involve digging up under our decking, laying a new pipe parallel to the back of our property and amending our existing full drainage connection. It is worth noting that we would not at any point be using the pipe that they will be laying - they would be merely joining into our main sewer connection
  • Our main asks are ultimately whether this is something we should agree to:
    • Who would be liable going forward should the rainwater pipe cause issues with our drainage (waste & rainwater), or cause damage to our garden if leaks started etc?
    • By agreeing to the new pipe across our garden & the connection, is there anything we need to be aware of/looking at in more detail from a legal perspective. Could it affect any future ability to extend our house or sell? Does agreeing to this work allow any future access rights to our garden?
    • Should we request a survey/inspection of what water flows into the pipe? We presume this, and any other costs, should be picked up by the neighbours?
    • What documentation would we need to get from them (and their builders) after the work (assuming we agree) has been completed?
    • We have seen online the ability for the neighbours to purchase insurance to cover for any damages to our property - is this something anyone has ever come across?
    • Is there anything else we should be aware of that we haven't thought about?
    • Alternatively, is this something we shouldn't accept, and should ask neighbours to find an alternative solution?

Thanks in advance for any guidance (and sorry about the length of the post!) - happy to provide any more details if needed!
 
I think you need to consult a solicitor. Before you do, think very carefully about whether you or a future owner might want to do building work that would be affected by the proposed new drain.
 
Along with any other actions, you need to employ your own Consultant at the neighbours or their builders expense, just as a party wall issue would be.
Also any solution that is agreed needs to be with the agreement of your Solicitor as above, again at the neighbours expense and would need exclusion of any future liability by yourself for any issues relating to the pipe and any other work connected with it on your property and full liability for the neighbour and any subsequence owner of the property.

I don't beleive that any new rainwater connections are allowed to existing combined systems, however there may be exceptions and as long as Thames Water are actually consulted and agree then the principle should be O.k, but the liability should still remain with the neighbour.

Indemnity Insurance cover is used for future issues to existing problems but I doubt you can use it to cover a proposed solution that may end up being an issue.
 
If the OP's sewer becomes shared with the neighbour, does the ownership of the shared section automatically become public? Something to ponder with the solicitor...
 
I would take some legal advice before agreeing to anything much as you want a a good relationship with your neighbors their works should not in pact your lives or cause you problems, you may well agree in the end but it won't do any harm to consult a expert in these matters. Kop
 
Thanks all - I have been trying to find solicitors etc to provide an update/speak through the main areas, but it it proving difficult!

As an update, Thames Water have come back and indicated they do not have record of the rainwater pipe at all - so want to do a survey/camera view of the two drains. For these, they need to access the sewers via our property, which we are agreeing to (on the caveat everything is returned to as it was, and it is at no cost to us, and it is not an agreement to the connection to our system). We have also informed the neighbours that unless our concerns (above) are answered/mitigated, we are unwilling to accept the connection to our system.
 

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