Jump to content

Building over / near a main sewer (600mm)


Recommended Posts

Hi crew,


I'm one of those that lurk on the forum religiously but have nothing of value to add... so far.


Building over the typical (upto)150mm clay has never been an issue but this is the first time I've encountered a 600mm drain running through the garden.


I'm wondering if any one has experience in building over / near a main sewer.


  • SE had specified a strip foundation prior to discovering the main sewer
    • Currently looking at a cantilevered slab as an option if need be
    • Extension is to the rear and side (wrap around)
  • Method of construction
    • Specified as Brick and block
    • Considering changing this to timber frame (my preference) if need be
  • I've contacted Affinity water to discuss options
    • advised to fill out an application
    • send fee
    • await cctv survey

Whilst i await furter updates, i'd be grateful to hear your experiences and how you tackled similar situations so i have an idea of what to expect.


  • What changes did you have to make to your plans?
  • What additional costs arose?
  • What you'd do differently if you faced a similar situation again?


Any advice, insights would be greatly appreciated.



Link to comment
Share on other sites

We've moved them for housing estates but not for a single house extension. Essentially it's the same process but the costs are dramatically higher for bigger sewers - they are much deeper!


They might go for build-over (cantilevered) but it is not a certainty. Is there any scope to change the floor plan to avoid it?

Link to comment
Share on other sites

Thanks for your responses!



Apologies, you are correct - I meant Thames!


I believe 3702(mm) is the depth of the pipe.


@George As it stands I'm in two minds about the entire extension... I can stick solely with a rear extension and slant / step-in the rear right corner to avoid the 3m distance-to-sewer rule but to be honest I'd pull he plug on it if that was the case especially if I had to lose much of the footprint.


I'm not overly precious about extending the house but being that I have the ability and resources to do so, it just seems silly not to.


Has anyone seen / experienced raft foundations serving as a suitable solution in cases like this?

Link to comment
Share on other sites

Water companies will, by default, say no. As dealing with these kind of things is a PITA. You'll have just talk and badger them. A good design will see you through. Last thing you want is to divert, a good build over design is the only approach 

  • Like 1
Link to comment
Share on other sites

10 hours ago, mrmckenzie74 said:

I'm one of those that lurk on the forum religiously but have nothing of value to add... so far.

But you have now!


Below is a screen shot from the Sewer for Scotland guide, same pretty much applies all over the UK.


It's pretty clear that moving the sewer is not an option.. too expensive and where would you move it to? Also any type of deep foundation strip / trenchfill directly under the side extension could well compromise the existing house foundations due to it's depth.. you are getting into complex / costly Civil Engineering territory.


The manhole up the side of the house is a big issue as you need to maintain access.


However, this is worth a look at in principle. Say up the side of the house you put in two or three carefully bored piles near the boundary and then run beams / make a cradle from the gable of the house to the piles near the boundary. Your side extension then sits on part of steel cradle but avoids the manhole and doesn't extend over the line of the drain. At the rear, as you are piling anyway you just do the same maybe with a cantilever concrete ring beam.


This could work and reduces the risk that if something goes wrong the water board come along and demolish your extension. It would only be two steel beams that span over their drain which you could dig under. I would draw this but have clocked off the day job.


Sometimes it can work where you bridge over a sewer with a bit of structure (not a monhole) and negotiate access with the waterboard.


In the round though.. unless you have a very high value property then it's unlikely going to fly cost / benefit wise, unless you work for / own a piling company..


I see this quite a lot where plans get draw by an Architect.. the SE does a design based on the plans.. but no one does a bit of due diligence to check the drains! The professionals should be much more alert to this early on. But often a Client doesn't want to pay for this fundamental early work.


Some of the checks are really simple.. a site walk over, a walk round the surrounding area, topography of the ground and a service check.

10 hours ago, mrmckenzie74 said:

What you'd do differently if you faced a similar situation again?


Undertake due diligence! The sewer may even be mentioned in your title deeds.













Your drawing needs a bit more detail as dimensions of what you propose vs what you have are critical. In other words a decent plan existing and proposed plan is required.





Link to comment
Share on other sites

@Conor I'm a big fan, I followed your build and was very inspired... Inspired enough that I'll be starting my own (ICF) build next year - thanks for the content!!


@Gus Potter not sure if you remember me, but you've helped me before on a loft conversion a couple years back, thanks again for that and for your reply today! 


Your approach definitely makes a lot of sense but I hear piles and quiver! I think the ultimate hurdle - as you've mentioned - will be overcoming that manhole in the side garden. 


Currently the plans drawn and approved are for a 3.5m single storey rear which (by my estimates - not knowing exactly where the drain diverts across) encroaches on the 3m sewer distancing rule, by around 1200 - 1500mm.


The cantilever makes sense for that right corner and a lightweight timeframe would complement the build further.


I've only recently considered wrapping around the property, so nothing drawn up yet. I remembered seeing a manhole out there so dug out my drainage report and realised what I would be contending with.


I may just opt for a car port here to keep things simple or a garage if that's allowed.


Link to comment
Share on other sites

@Conor has done a lot of good work, detemined fellow.


50 minutes ago, mrmckenzie74 said:

not sure if you remember me

Honestly, I don't as you don't use your own name but thanks for the kind words. I love helping folk as my job is also my hobby. I do have a wife who loves me though!


I think you need to post some decent sketches with dimensions.


I was floating some ideas but until you put some flesh on the bones there much more I can say.  To be blunt I love a challenge. I'm on BH as I learn a lot.. from loads of folk on BH,  it is a two way street.


Look you need to get to the bottom of the drains / sewer and start to do a bit of the hard graft at your end. Stop speculating about cantilevers etc and find out what the water board will let you do and what they won't. I'll design you cantilever slabs all day long.. but not now or give you advice on this until you get the basics sorted.


Frankly I'm surprised that you have got this far and non of your designers have said let's see what you have under the ground. I often say to folk.. I want to have a look at your title deeds.. Sometime I do this and the project gets abandoned at an early stage.. but these customers always come back to me as I have saved their bacon.


Can you get your money back from the SE and designer? How did this happen where you got this far?


How much money do you have to spend on this?  Draw it out for your self and you'll soon see it it tricky and expensive. 


I may be sounding a bit tough but you need to confront the elephant in the room.. which is this big drain. Look closely at the drain diagram I posted and study..






Link to comment
Share on other sites

Agree piles and cantilever beams the most likely viable solution. Rafts wouldn't be suitable as although they wouldn't damage the sewer, the water company is more concerned about getting access to the sewer in future and they wouldn't want to undermine a raft.

  • Like 1
Link to comment
Share on other sites

22 hours ago, mrmckenzie74 said:

any one has experience in building over / near a main sewe


But our client had to sign a document that allowed the water company to dig up his floor if necessary for maintenance.

We designed his warehouse to allow that without losing the building.


Ask them but expect the answer to be no.


Link to comment
Share on other sites

20 hours ago, mrmckenzie74 said:

I believe 3702(mm) is the depth of the pipe.

No - it's a reference number for the chamber - you should also have a list of the nearby chambers together with their cover level and invert level and hence their depth. Not is all lost - I once dealt with a rear extension that had a 1500mm dia surface water sewer running up the shared driveway between the applicant and his neighbour in St. Albans. It took a while to sort out and cost him more than you'd want but not silly money. Approval was given by TW and it got built.

  • Like 1
Link to comment
Share on other sites

@Gus Potter I'm here to learn too. Me asking these questions (speculating as you say) is educational for me. The term "cantilevered slab" is a fascinating concept to me, I even started to explore the possibility of a cellar over the chamber - which again, as a concept is exciting as hell to me - be as blunt / tough as you need to be, I'm just grateful you guys are taking time to share your experiences with me and I genuinely love this sh1t.


As mentioned -I'm in the process of getting the basics sorted. I'm being as pragmatic as I can as I've never been here before.


TW have a set of drawings and outlined the application process - I assume that TW will need another set of drawings with revised foundations and doing my homework means I should be able to present them with the best options.


I'd not dream of asking you to design anything at this point, this is just a preemptive discussion.


Budget - £50k

Self built so I'd expect a fair bit of change from this. 


SE has done nothing wrong as his scope was (and still is at this point), a rear extension. As mentioned above - "I" am enquiring about the wrap around.


Good shout on the title deed thing too, although I've never had an SE as thorough as that but i'll be sure to give you a shout first in the next one.

  • Like 1
Link to comment
Share on other sites

@George Very helpful response, these are the things in trying to get my head around.


- Can I move the sewer, probably but it'll cost a bomb - so No!


- Can I build over the chamber and or sewer, No access must be maintained, raft not likely to be suitable here.


@saveasteading Brilliant! whilst this isn't a green light, it does open the door of thought for a garage / gym with access to the chamber. They may say no, but once I've had the survey I'll definitely be asking the question. 


Thank you both very much 😊 


Link to comment
Share on other sites

Yes, and not without issues


A large extension I built had a 600mm sewer at a depth of 6M across the corner of a proposed extension.

United Utilities agreed to a build over, with a piled foundation, ( seems like a must as the ground is backfill). The customer needed a sewer camera survey before and after works, in my case the later was when the issues started as 1 pile cored the edge of the pipe.  Luckily the piling contract was direct with the client and bypassing me.

Stress full time.


Link to comment
Share on other sites

29 minutes ago, mrmckenzie74 said:

build over the chamber

It would have to be a sealed cover, and I can't see it being allowed in a habitable space. Garage maybe. but don't get excited.

99% they say no.


Link to comment
Share on other sites


Ahhh thanks for the correction! 

I don't have a list, only the map that I have sent you. I assume that there's a more detailed report so i'll look in this.


Whilst fantasizing about creating a cellar I paused at the thought a 4m deep dig out, considered underpinning and muck away then realised that's a lot more than I want to do on this project, so hopefully it's not that deep.



Great to hear that a solution was found how large was the sewer and did your work around add a considerable amount of time to the scheme?

Link to comment
Share on other sites

On 19/04/2024 at 17:54, mrmckenzie74 said:

I'm here to learn too.

Lots to learn indeed, every day is a school day for me too!


These jobs require careful thought, not just the engineering challenges. When you come to sell is a potential issue. Who want's to buy a house, part of which could be potentially demolished at the owners cost for the water board to access the drains? You may get build over permission as discussed above but it comes with a caveat as you can see from other posters.


If something goes wrong with the sewer then you will suffer disruption and possible high costs! One way to mitigate this is to look into taking out an indemnity policy to cover yourself now.


It would be worthwhile researching the cost of such a policy. They tend to be a one off payment and the policy gets tacked onto your deed paperwork.









  • Like 1
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Create New...