Neighbouring property draining to our private manhole

[Kiaara]

We have started building a side extension to our house within London and discovered that a manhole (which will be moved as part of the extension) in our garden is being used by the neighbouring property to drain rainwater. This is not a public manhole and does not appear in the public sewerage map. Our extension is now blocked due to this issue. How should we proceed to resolve this?

What is the exact arrangement? External drainpipe?

What does it connect to? Soakaway or foul sewer?

@Roomgigi neighbours gutters -> neighbours manhole -> our manhole -> public sewer

@unsync neither, it connects to the neighbours manhole which in turn accepts surface rainwater

As far as I’m aware rainwater shouldn’t be entering the sewage system, but tricky as both your houses are doing that!
I’d imagine both of you will need to sort a new soak-away.

Does your neighbour's sewer fully drain into your sewer or is it just rainwater? If the former, and your manhole is for the inspection chamber for rodding etc, then you probably won't be able to build over it. You need to speak to your water company. We had to dogleg our extension for this reason.

It might be a public sewer as the maps aren’t very accurate

@user5972308467 all the houses in the neighbourhood do that as the clay is too hard for soakaways. But thanks anyways!

Most London sewers are combined surface and foul water.

If your neighbour's sewer connects to that manhole then it is part of the Thames water network. You will need a build over agreement with Thames water to move that manhole.
The new arrangements must allow existing drainage from the neighbour to continue.

You might be able to get your neighbour to agree to move their drainage if it is blocking your extension from being built. But they don't have to agree.

A thorough survey of utilities is important when planning extensions.

@DaveWatts it’s just rainwater and to be precise, the neighbour has their own foul sewer and so do we. This manhole just accepts rainwater and drains it into the public sewer

Only correct answer so far.

Edit to add: water companies took over responsibility for pipework shared by more than one property some years ago. So the bit of pipe that is shared is no longer private. You will indeed need a build-over agreement. My neighbour was allowed to build over pipework and a manhole cover but wasn't allowed to run a wall along the length of pipe (but was allowed to put a wall across at right angles.) That was not in the Thames Water area though, and I expect all water companies are a bit different.

@Rollercoaster1920 they have separate foul sewers and so do we. This is just rainwater.
We did procure a Thames Water map which doesn’t show this.

The maps won't show these smaller parts of sewer. Rainwater or foul doesn't matter, it is all waste water discharge.
Watch the videos:
https://www.thameswater.co.uk/help/home-improvements/ownership-of-private-sewers-and-pumping-stations

Doesn't matter. We just went through a not too dissimilar situation; sewers of this kind became part of the water company's network after 2011. If it drains to the public sewer, you will need both a build over agreement and also an agreement/inspection to certify that whatever work to redirect it is up to scratch. Penalties for not doing the latter can be massive.

Edit: this comes under section 106

www.thameswater.co.uk/help/home-improvements/how-to-connect-to-a-sewer

does not alter the fact it is a shared drain and therefore the responsibility of Thames Water since it is integrated into the "public" network

• build over agreement from TW required