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Extension 'Build Over Agreement' with shared sewer AND lateral drain

Discussion in 'Building' started by GeeTee123, 4 Mar 2019.

  1. GeeTee123

    GeeTee123

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    Thanks in advance to any of you experts that can give their thoughts on this.

    So, we're looking to do a single storey extension at the back of our terraced house - this is complicated a bit by the fact that a build over agreement will be needed to deal with a shared sewer for neighbouring properties (which 'terminates' here by turning, at a manhole, and travelling under our house to the main sewer).

    I've done a good bit of research around this, but I've not found any examples of work like this where there has also been a lateral drain joining at the manhole (that is: our neighbour's drain doesn't join the shared sewer before it comes to the manhole - instead the drain comes direct to the manhole).

    Here's my rough diagram as explainer (including my first version of a possible approach to dealing with it):
    sewers-y-v1.2.png

    (The angles are approximate, and the left and right edges are the boundaries with neighbours. I'm pretty sure of the connections though).

    There's a concrete patio covering this area currently, and we're on a tight budget so we'd really like to pin down whether we're going to be able to get Thames Water agreement before we get as far as opening up to lay down foundations. So far our initial chats with possible architects have not been very useful on this aspect of the project.

    Clearly the current manhole will need to be removed, and Thames Water have been able to go so far as to say a Y junction to get a replacement in would be the way.

    ...but is that going to suffice if the drains for both ours and our neighbours also join at that point?

    ....if not, will they perhaps be OK with the neighbours lateral drain being swept up to instead join at the new inspection chamber (with ours also being taken out to join there instead presumably?

    Architects and Thames Water have all sounded fairly positive so far, but none of them have actually been in a position to confirm how the sewers could actually be dealt with!

    Perhaps we're just going to have to bite the bullet and pay up for a Build Over application before we even proceed?

    Thanks in advance!
     
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  3. ^woody^

    ^woody^

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    What's the actual question? If you get the build over agreement then all is well. You will still need to allow the neighbour's drain to drain across your property.
     
  4. GeeTee123

    GeeTee123

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    Well all the architects have suggested they'd leave this details until the building regs plans are done and/or would leave it to the builders when the site's opened up, but as I say we're on a tight budget so we hope to pin this down before the thousands needed to get that far have been spent. We want to avoid getting there then it proving impossible to get a build over agreement.

    Do either of my two speculated approaches to it seem likely to be acceptable (in terms of access for rodding, turns in bends, etc. etc. that Thames Water would want)?

    ...but yeah as I say perhaps it's actually the case that I'll just have to go ahead and put together a Build Over agreement application by myself in the hope of securing approval before we even get the professional plans and build regs plans produced.
     
  5. Ian H

    Ian H

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    I’d say the manholes too busy to build over.
     
  6. Notch7

    Notch7

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    Ive dealt with Thames water, they are quite helpful, although difficult to get through to the engineers on the phone.

    I wouldve thought thought the existing manhole would need moving to somewhere around where your writing 'proposed extension rear wall' is on the drawing.

    you will need: cctv before work commences, agreed specification for works, then cctv after work done but before all backfilled.

    The job I did needed to be a brick built manhole and clay pipes where the existing public drain was moved -plastic for all private connections.

    What you will need to do is send an application and drawings to Thames to get approval -they might tell you what can be done by discussing with engineer, then you can talk to you builder to get some idea of cost.

    Chicken and egg unfortuantely!
     
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  7. Notch7

    Notch7

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    I wondered that -would it work to be moved?

    -I suppose it depends on invert levels etc
     
  8. ^woody^

    ^woody^

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    There are too many connections for a Y. The issue is whether the water Co. allows internal manholes (see their policies). If not you will need to divert the drains, and then the issue becomes whether you can get on to the neighbours property to instal new chambers and move pipes.

    Building control will have a say over the drains that don't belong to the water company, so may apply a different policy.

    Your designers should be detailing this as part of their design work.
     
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  10. GeeTee123

    GeeTee123

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    Thanks all. Thames Water don't allow internal manholes. The neighbours have already extended over, meaning there's no scope for moving much (well, anything really) on their side. They did have to put a Y junction with an inspection chamber in themselves though so I do know that there's inspection chamber access immediately upstream of the shared sewer (not the lateral though, frustratingly. If the two had been merged on their premises it'd be a different prospect!

    "I wouldve thought thought the existing manhole would need moving to somewhere around where your writing 'proposed extension rear wall' is on the drawing."

    - Is that because it needs to be a certain distance from the current one @Notch7 ? From what I've read in the Thames guidance it'll need to be at least half a metre away from the boundary and the foundations.

    "I’d say the manholes too busy to build over."

    - I'm hoping though that if they won't allow building over it 'as is' @Ian H then we could instead take ours and the neighbours drainage out to the new manhole and then the existing manhole location would only be a single pipe on a bend (and accessible from that new manhole).

    Hmm, it sounds like I'll definitely need to pursue this with Thames Water first then.
     
  11. GeeTee123

    GeeTee123

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    That's another detail rather obstructed by the darn concrete patio. The invert at the current manhole is approximately 1.5m but I've no way of determining what it is where the sewer and lateral pass on to our property so as to determine the fall there.

    There is an inspection chamber where it joins the mains at the front of our house though so I could theoretically calculate the fall under the house I suppose (that inspection chamber is really rather deep so it's a decent fall under the house I think).... though I'm not sure if that'd be useful for the calculations.
     
  12. Leofric

    Leofric

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    How many architects have you contacted ? Safest to check you can make the drainage work before going too far.:cautious:
     
  13. ivixor

    ivixor

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    Pretty sure this will be a big fat no from the water company, for multiple reasons.

    - No manholes inside building
    - No changes of direction without manhole
    - No connections without manhole
     
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  14. GeeTee123

    GeeTee123

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    We've been in contact with 4 now (providing them with some details and mention of the build over position), though only one has done a site visit. Unfortunately none have been particularly committed to dealing with the build over first, and all were fairly casual about it. If we leave it til the building regs/structural drawings stage we could already be as much as £3k down the road to then find out it's impossible!
     
  15. GeeTee123

    GeeTee123

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    Yeah you may be right. In response to an initial touch-base with Thames Water they did indicate they'd in principle accept a light bend in the direction so as to avoid a manhole under a building though. My Plan B looks the most likely, with the connections then being made to the new manhole.

    Thanks for your input all!
     
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