Building over a sewer

[Russell Beer]

Hi Guys,

First post and thank you in advance for any advice given.

Ive had an offer accepted on a house and the plan was to extend to the side to create a kitchen dinner and put a utility in the back of the garage.

It all looked very simple. However, I have just had the searches back and there is a public sewer running under the garage and what looks like the corner of the house. The house was built in 1966 so I doubt there is permission to build over the sewer.

The proposed extension would be going over the sewer further and also adding a utility in the garage will be building over the sewer.

The sewer is 470mm clay pipe (combined sewer), depth of approx 3.87m to invert. Owned by severn trent.

Before I start putting more money into CCTV survey, electro location survey, detailed plans and the build over application I would like to know:

1. What is the likely hood of severn trent giving permission to build over?
2. Has anyone had experience with this before?
3. If severn trent were to give permission to build over is the build cost going to significantly increase due to the sewer?

I have attached current and proposed plan of the house and marked roughly where the sewer runs.

Also attached plan of sewers from severn trent. Plot in question highlighted in yellow.

Thanks

 

[^woody^]

You won't get an answer here as there are several variables. But ST are generally pragmatic and accommodating.

I'd be more concerned about the practicalities of having to dig down 4m for the foundations between the two properties. Or just allow for the cost of piled foundations.

You will need a proper foundation design proposal to submit with the build-over application.

 

[Russell Beer]

Thanks for the reply. Really appreciated.

So the foundations would have to be deeper than normal due to the sewer or would the foundations have to be 4m anyway?

Would piled foundations be a lot more expensive?

 

[^woody^]

You won't be permitted to put the weight of the building onto the ground above a drain or sewer, as the thinking is that it could crack the pipe. So near pipes, foundations have to go down at least as low as the pipe. Normally foundations would be 1m deep.

I don't recall what ST''s policy on raft foundations is. If allowed, this could be a better option.

Piled foundations will be several thousand pounds more than 1m strip foundations.

 

[Notch7]

I know Southern water wont allow anything to be built over a sewer pipe more than 350mm dia or over 3 metres deep (from memory, is was a while back I looked into it).

450mm is a major public sewer, you certainly cant assume they will let you build over it.

Maps can be wrong, so I suppose you need to know where the drain actually is, as a starting point.



I know somebody that started building an extension, then discovered a drain 3.2m down, it wasnt obvious because there were no manholes in his garden only adjacent ones.

 

[Russell Beer]

Thanks for the info. Really helpful.

Just wondered if either of you know if I was to change the plans and extended the dinning room.

Which will not be building over the sewer but say within a meter of it.

Will I still need permission from STW?

will the foundations still need to come down to the same depth as the sewer or will 1m foundations be fine?

 

[Notch7]

Thanks for the info. Really helpful.

Just wondered if either of you know if I was to change the plans and extended the dinning room.

Which will not be building over the sewer but say within a meter of it.

Will I still need permission from STW?

will the foundations still need to come down to the same depth as the sewer or will 1m foundations be fine?

The term build over is misleading, it should really say build near or over agreement, because you need to apply for one if building with 3 metres of a sewer.

You will need foundations deeper than invert. A rule of thumb is that the pressure from the bottom of a foundation goes out and down at 45 degrees, both sides.

 

[Russell Beer]

So if it’s within 3 meters the foundations definitely still need to go deeper than the invert?

Surely if it’s close to the sewer and not over and the foundations go at 45 degrees then it’s getting closer to the sewer which can’t be good?

 

[Ian H]

It’s called the angle of repose. Imagine a line coming off the bottom corner of your foundation at 45deg downwards, that line needs to go under the sewer.