Build over foul sewer - concerns

[marsh3377]

Hi all,

 

I am wondering if anyone can offer any help, advice or pointers.

 

I am looking to purchase a plot which has a foul sewer pipe going diagonally across it. In order to make the most of the plot I would like to build over the drain as per the diagram below. There are inspection chambers (the red circles) either side of where I am proposing to build over (proposed house outline in blue), so access should not be an issue.

 

 

 

The reason for my post is that even though I know a buiid-over is possible, I am concerned whether it will be more hassle than its worth. Just to add to the situation, due to the water table and soil conditions, piling is going to be needed (down to 18 or 20 metres).

 

So, my main areas of concern are:

 

1) What are the chances of the water company just just refusing to allow a build over agreement?

2) The pipe being damaged by the piling vibrations

3) The requirements (and additional cost) for calculating, designing and actually installing the piles

4) The negative effect this may have on any potential sale (as it will come out in a survey)

 

I am therefore wondering whether any structural engineers or anyone else who have had to do similar on their projects can offer any any wisdom, advice or any other extra information (either positive or negative!).

 

Many thanks!

[TommoUK]

Looks like a public sewer rather than a private sewer so you will need a build over agreement.  You will need this even if you don't actually build over the line of the sewer but are within 3m of the sewer. 

 

Building within the 3m zone should be ok, and you may be able to self certify the build over if you comply with certain rules,

 

they don't like you building a new house over the actual line of the sewer unless there is an existing house there already, so you may need to move your building back in the plot or step back the part of the frontage the crosses the sewer

 

 

[marsh3377]

Thanks for your reply, appreciate it.

 

When you say "they don't like you building a new house over the actual line of the sewer unless there is an existing house there already" - are you saying there's a very good chance they would refuse a build over agreement?

[Dave Jones]

3 minutes ago, marsh3377 said:

Thanks for your reply, appreciate it.

 

When you say "they don't like you building a new house over the actual line of the sewer unless there is an existing house there already" - are you saying there's a very good chance they would refuse a build over agreement?

 

they will make you divert it, if there is fall.  Very very unlikely they will allow it through your footings.

 

 

[Conor]

A diversion would be required with a new manhole being needed in the public highway. Budget somewhere around £5k minimum. Maybe £10k if the rest of the section needs to be modified. But there a chance the records are wrong, it certainly looks a little strange, worth lifting a couple lids to check.

[marsh3377]

On 24/08/2024 at 17:14, Conor said:

A diversion would be required with a new manhole being needed in the public highway. Budget somewhere around £5k minimum. Maybe £10k if the rest of the section needs to be modified. But there a chance the records are wrong, it certainly looks a little strange, worth lifting a couple lids to check.

 

I've read on here and other places that going down the diversion route should be avoided if possible but from what you're saying it doesn't sound too bad?

 

When you say it looks a little strange and the records may be wrong, in what way are you thinking?

[Dave Jones]

if the sewer isn't that busy when you lift the lid, id be tempted to just put a chamber in myself to divert it around the footings and then plead ignorance on the buildover application. 

 

Or just pay the 10k and get it done in the road ?

 

All depends on your risk appetite.

 

[DevilDamo]

Why does the proposed dwelling need to be that shape? Easier to re-design the dwelling as opposed to trying to request a diversion. 

[Marvin]

Hi @marsh3377

 

Personally I would want to know how many properties are connected to the manhole at the front, how deep the manhole is in the ground and how big the diameter of the pipe is.

 

For your information recent discussions I have recently had indicated that the 3 metre distance is measured from the nearest part of the building footings to the nearest outside edge of the foul pipe, not from the external brickwork face to the centre of the pipe. There were also discussions about the type of ground and the 45 degree angle of load bearing from the bottom of proposed foundations. 

 

in the picture below I do not think that the footings would be permitted because they would put a load on the sewer pipe (although saying this it is possible but not without further protection... complicated. You need to know how many properties are connected to the manhole at the front, how deep the manhole is in the ground and how big the diameter of the pipe is, and this information will give you some indication of the challenges.

 

And finally as @Conor said above the records saying where the pipe is may be wrong. At the back of the properties down our side of the street the main sewer is 4 metres nearer to the buildings than the records show! That the difference between planned positions and "as built"!!!!

 

Good luck

 

Marvin

 

 

 

 

 

 

Edited August 31 by Marvin
clarification

[twice round the block]

You say piled footings of 18 - 20m may be required.

Is this plot / build viable? 

 

How deep is the main sewer? 

 

 

 

[TommoUK]

On 24/08/2024 at 15:09, marsh3377 said:

Thanks for your reply, appreciate it.

 

When you say "they don't like you building a new house over the actual line of the sewer unless there is an existing house there already" - are you saying there's a very good chance they would refuse a build over agreement?

Yes...and you will still need a build over agreement if you are building within 3m of the sewer.

[Mr Punter]

While you are gathering the information on the sewer (diameter, invert, material), find out where the water and electric is to make sure there are no clashes.  It looks like you could set the proposed house back a fair bit.

[marsh3377]

Thanks for everyone's replies.

 

Re the plot itself, the water table is very high and therefore the ground wet, hence needing the deep piles. It is a viable plot though, with houses either side of it.

 

As for changing the shape of the proposed building, I don't want to do that because I need to maximise the financial viability and this shape best achieves that. Also, I don't want to push the house back as it's likely planning will be refused due to both moving off the current build line and affecting next door's light.

 

I have now had confirmation from Thames Water that I do indeed need to divert - they will not sanction a Build Over agreement for a new build. It doesn't seem like a very big diversion is needed though, as per the green dashed line below. The question is what sort of cost this will be.

 

[ProDave]

Get a quote for the diversion.  That is the only way you will know.

 

High water table does not always mean piles.  Our water table here is high in winter, but normal strip foundations about 1.5 metres deep are fine.  You just need to get through the soft organic top soil into something solid.  A SE witnessed a number of test pits dug down to about 2 metres and confirmed all was well.  Our previous house nearby was the same except it needed to be a little deeper as that plot the ground had been made up a bit.

 

The most cost effective build shape is a square box, not an L shape.  If you could mirror your L diagonally so the back of the house was a straight line and the notched out L was at the corner where the sewer was you might be able to avoid the diversion.  And moving it back on the plot just a bit would also help.  I can't see planning objecting as the houses to the north (assuming north up) are set well back so I can't see that as a problem.

 

Finding the exact location of the sewer will answer is it possible to design around it and avoid the diversion or not.  That would be my priority.

[marsh3377]

Thanks all.

 

I have got a couple of companies coming back to me with quotes for moving the pipe.