Extension Dilemma - Parallel Running Sewer

[Carmellington]

Good morning all!

I have a big dilemma and I do not know which way to turn, so would love some opinions/advice. Our semi detached house has a public sewer (shared by us and our neighbours) that runs right to left before heading down to the road from our driveway.

We currently have a kitchen extension which has a conservatory attached to it (this was already here when we bought the house). Our plans are to demolish the conservatory and to rebuild it, put a pitched roof across the whole width and open everything up internally using steels and then have bi-folds along the back wall.
The issue is that our Architect says the public sewer running parallel is too close (within 500) of the existing kitchen extension and conservatory wall and that we would need to either come in or go out so that we have enough clearance. This obviously would increase our build costs massively as we wouldn't be able to keep the existing kitchen corner foundations. Going out would be easier to swallow, but paying all of this money to come in and actually reduce our current footprint seems unbearable.

We met with a load of builders who have all said that we would be fine keeping the existing footprint, which is ideal, but the Architect believes we wouldn't get sign off....

Has anyone encountered anything similar to this? Is it advisable to look at possibly diverting the sewer out slightly to give us the 500 clearance required? Is there a chance the water company/ building regs would allow a build over agreement or something similar?

I've attached pictures to help you visualise. But any advice/ recommendations would be amazing.

Thank you!

 

[Carmellington]

Forgot to add this

 

[freddiemercurystwin]

Which water authority are you with?

 

[Carmellington]

Southern Water

 

[RonnyRaygun]

One option is to put a foundation either side of the pipe, ensuring you maintain sufficient distance from it, and then bridge over with a concrete slab and build your wall off that.

Here's one I prepared earlier:

 

[freddiemercurystwin]

That drawing is of the existing as it is now presumably? How far is the drain from the foundation currently then? How far is the drain from the inside face of the wall? An eccentric (or offset) foundation can give you a few extra inches. Southern Water's rules do state 500mm from the edge of any foundation https://www.southernwater.co.uk/media/4882/build-over-a-sewer-guidance-notes-v30.pdf page 9 & 11.

 

[Carmellington]

Thanks guys for your replies. @RonnyRaygun in your experience, is this something that Building Control/ Southern Water would find acceptable?
@freddiemercurystwin the drawing in the photo is the existing yeah. I've attached a photo showing where the pipe runs under our existing footprint. As it stands the pipe is 627 from the corner, but obviously we need to account for the foundation width as well as the width of the clay pipe.

 

[RonnyRaygun]

Thanks guys for your replies. @RonnyRaygun, in your experience, is this something that Building Control/ Southern Water would find acceptable?

I can't see why not as it complies with all of the requirements of a build over agreement. It was acceptable to BC and Thames Water when I provided that detail to them a year or so ago.

It's not really any different from building over a sewer with your extension and a ground bearing slab or beam and block floor over it. You'll probably also need to provide a new like for like inspection chamber to allow for rodding access if you're also building over an existing inspection chamber.

Southern Water should be able to give you some advice on what they'll accept prior to you actually sending in your proposals.

 

[freddiemercurystwin]

Isn't there already a foundation under the whole back wall? In which case you're not changing anything/making anything worse are you? You can move the pipe run but you would need the consent of the neighbour, more cash though of course. Ronny's soluion is fine structurally, depends on the dopes overseeing the application though, but I suspect less expensive to build the whole lot bigger.

 

[Carmellington]

The foundation on the Kitchen section of the run is fine, however the conservatory was tacked on at a later date, trial pits suggest it needs better foundations and the Architect doesn't want to put any additional weight on the existing.
I guess the only issue with making the whole lot bigger is that we couldn't keep the Kitchen bit of the run which would add to the build costs, it would also eat into the patio area we are looking for :/

 

[freddiemercurystwin]

Space-wise you're only talking about moving the whole back wall out by about 150mm with an eccentric foundation underneath, how much is a 2.5x2.5m panel of cavity wall and foundation anyway in the grand scheme of things?



Probably less expensive than any alternatives anyway.

Some water authority's are little bit more flexible than others and will engage in some discussion/negotiation, I have no experience of Southern Water so can't comment there, your 'architect' should be going through all this stuff rather than just saying 'computer says no'!

 

[Carmellington]

Thanks again for your replies. I just read this on Southern Water which I believe may put a spanner in that idea?

• The pad, raft and cantilever foundation types over the public sewer are not permitted as it can impact our future access to repair the adjacent public sewer and significantly increase the risk of damage to the structure during repair works. We also do not permit reinforced concrete floor slabs over public sewers for the same reason

 

[freddiemercurystwin]

Well the eccentric foundation doesn't do that and the drain is just as inaccessible now than it would be using Ronny's design.