Moving drains running near boundary

[SouthLondonCharlie]

I'm planning a rear extension to a modern (1990s) terraced house in South London. I want to build a new wall along or close to the boundary with my neighbour, but the drains from both our houses run under the boundary. I've been told by an Anglian conservatory salesman that it won't be possible to build an extension there as the drains get in the way of the foundations, but I can build a consevatory...

Not sure I believe him, and I've heard differently from an architect who suggested I might be able to reposition the drains or alternatively put in a ground beam or a cantilivered slab.

Any advice from those familiar with the situation would be much appreciated.

thanks

 

[jeds]

The architect is right. (not very often you get to say that) You could easily build off a cantilevered slab. If the drain does serve both properties then it is almost certainly a public drain owned by the local sewer company. You will need their permission to build near/over it. Download details of build near or build over from their website.

 

[Newboy]

If it's a drain serving more than one property then it may be a public sewer (depends on where it runs to/from and how many houses) - if it is serving more than one house then you will need the permission of the local water authority to 'build over' it - a build-over agreement. In you case it will be Thames Water

https://www.thameswater.co.uk/developers/693.htm

 

[SouthLondonCharlie]

Great, thanks for the advice. There is a public drain/sewer running horizontally through the back gardens behind the terrace. The drain running from my house to that is private according to Thames as they don't have it on their plans. My neighbour's drain is either right next to it or they might join before meeting the horizontal drain (in which case Thames might be responsible for the common bit), but I I've yet to get the drains surveyed to check.

It's great news a cantilevered slab could support the extension wall. I hope that wouldn't add too much to time or cost.

 

[Hugh Jaleak]

Rules for drains are now fairly simple. If its shared, or its your neighbours, from where it enters your property, its Thames responsibility.

 

[SouthLondonCharlie]

Thanks, it does look like I'll need permission from Thames. I think that's a given anyway as I need build near or over the drain serving the whole terrace as well as repositioning a manhole. What's a bigger concern at the moment is that the runs serving my house & the neighbours are where my extension foundations need so be, so I either need to move those or build over with a cantilevered slab, ground beam or some other clever engineering solution. One builder who visited my garden said I simply can't build there, but I'm hoping that's wrong.

 

[Hugh Jaleak]

Provided an adequate fall can still be achieved, moving the drains is usually straightforward, just means additional digging.

 

[SouthLondonCharlie]

Here's a sketch of the drain & sewer run and a photo of what's under the manhole. The depth at manhole is 0.7m and the distance of my run from boundary is 0.5m. The neighbour's drain crosses the boundary into my garden (becoming Thames's responsibility?) and joins at the manhole. Thames tell me I will have to move the manhole.

Any tips on how this might affect construction of a new party wall along the boundary would be much appreciated. My plan is to build a 3m rear extension with new party walls along the boundaries. I have spoken to a couple of architects but what I currently lack is advice from an engineer/builder who'e dealt with this kind of setup.

thanks

 

[jeds]

If you are building full width you've got a tricky one there because you won't have anywhere to move the manhole to. The only solution I see is to create a new manhole on your neighbours land and reverse the connection - so yours crosses the boundary and connects on his new manhole. What would your neighbour think about digging up his prize dahlias for a new manhole?

 

[SouthLondonCharlie]

Thanks. When I spoke to Thames they said I could use a Y-junction to move the manhole, but they haven't seen the drain run so I don't know if it would work. I had a look at the download you recommended and it includes an example of one (pasted below). In this case the new manhole isn't over the junctions, so presumably that's OK...

 

[Nige F]

If you are building full width you've got a tricky one there because you won't have anywhere to move the manhole to. The only solution I see is to create a new manhole on your neighbours land and reverse the connection - so yours crosses the boundary and connects on his new manhole. What would your neighbour think about digging up his prize dahlias for a new manhole?

Everyone has their price

 

[SouthLondonCharlie]

The neighbour is OK - I'm pretty sure she would allow a bit of digging. She doesn't live there currently as the house is rented out, so the inconvenience would be to her tenants. I'd have to make good after the works though.

 

[SouthLondonCharlie]

After a quick chat with Developer Services, I think I can do the following. The drains are shallower than the foundations, so I think I will need to use cantilevered slabs/concrete lintels over them and build around.

Thames seem happy for junctions to disappear under the building as long as they can get rods down from manholes.

 

[Hugh Jaleak]

Your problem is going to be maintaining rodding access to the laterals. I can see them asking for the current run to be 'dog legged', from the neighbours garden, into yours, then back again into the downstream neighbours garden. Unless the rules have changed drastically, bends are only allowed immediately outside of a manhole, in your drawing you're asking to bend the incoming drain 45° under the extension, and connecting 2 laterals, then running it outside to turn 90° in the new manhole, to come back under the building and turn back 45° again onto its original line.

This is leaving both laterals inaccessible, unless a manhole is fitted on the run in the neighbours property and a rodding eye on your property, internally. Fine, until you come to need to use it..... Personally, I'd be surprised if the allow your current proposal.

 

[SouthLondonCharlie]

Thanks. The revised drawing is what Developer services suggested to me, but I might not have fully understood. Now e-mailed to them and I expect a call back within 5 days to discuss it and work out a solution that is worth formally submitting for permission.

Fortunately I get on with my neighbour so if I have put a manhole in her garden, I think that would be OK. The trickier neighbours are on the other side, where there are no drains.