rear extension - move drains + build over agreement

28 Dec 2021
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United Kingdom
hi all,

I need some advice on what options I have here. attaching picture for clarity.


long story short, I have 2 square manholes in my rear garden.

* one is around 1.4m from the rear wall (M1), the second one is around 4.15m from the rear wall (M2), as per image.
* M1 collects my rainwater, soil pipe and kitchen/bathroom water through 2 gullies and 1 soil pipe. it also receives rainwater from the neighbour.
* M2 collects everything from M1 along with a lateral pipe from the neighbour, which is probably his drain.

from what I understand and from the CCTV survey I had, everything beyond M1 (including M1) is a public sewer. we are looking to build an extension spanning the entire width of the house, 4m deep, as per image. this means I'll be building the foundations over two pipes: rainwater pipe from neighbour into M1 and the pipe connecting M1 and M2.

a couple of questions:

* is it correct that we're looking at public drains and hence I need a build over agreement or a self-certified building agreement?
* what options do I have in moving M1? clearly the manhole can't be under the extension, hence I'll have to move things around.
* what about M2? can M2 moved about 50cm further away towards the main? that would help me comply with 500mm distance rule.
* do I need 2 manholes? can the rainwater pipe be diverted straight into M2 so I'm left with one manhole only?

anything else I need to take into account?

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You should be able to build over m1, whoever deals with your drains won't like it but when there's no other practical option they should allow it.

Everything should be roddable from m2, if your redirecting your outlets as part of the extension you could look to remove the m1 completely, put a smooth curve on your neighbours drain and feed it to m2 where it should still be cable of being rodded or jetted
so basically add a bend in the neighbour's rainwater drain, somewhere under my extension, route it directly to M2. with that done, M1 basically becomes a private drain, as it only collects my stuff and I can easily move it. would it work if I created a new manhole just outside the extension and fed that into M2? basically move all my pipes (soil stack, rainwater, kitchen/bathroom waste etc.) into a new manhole that I create outside the extension and connect that new manhole to M2, but from a different direction.

also, would I be allowed to move M2 a little bit further towards the back garden? if I'm not mistaken, the public manhole has to be at least 50cm from the extension. if I leave it where it is, it will be just 15cm...
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Within reason you can move them where you like, you'll need a build over agreement regardless. The limiting factor is usually cost.

When we did our drains they were 2m deep at one end so that limited the options as you don't want that size trench open for long, when severn trent inspected them the bloke said he wouldn't of dug that far down he'd of fitted a back drop which severn trent say they don't allow but the bloke said they have to allow for practicality of the options available whether they like it or not.

The only issue with moving m2 will be getting the drain running laterally into it sweeping nicely if your not moving it far.

If you combine some of your outlets into a bottle gulley you can probably get away with running everything into m2 wherever you decide to position it, a manhole tray normally has 5 inlets

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