building within 3m of public sewer

Hi,

I would appreciate some advice, i have planning permission for a single storey rear extension under permitted development, It will extend 6m from rear of house and i intend to do most of the work myself, with the help of family.
I have a bricklayer, plumber and sparky within extended family and i am a roofer, Albeit industrial, but roofer none the less.
What i am missing is someone with knowledge of the below ground works.
Upon submitting building control notification i found that i have to contact the water authorities to check that the development is not within 3m of a public sewer, which i did only to find that i have a public sewer running across the width of my rear garden, it's far enough away that it isn't within 3m of the extension once complete but this sewer turns 90 degrees inside my neighbours garden and heads towards, and under her property where it meets the main sewer in the street out the front.
The plan showing the sewer shows a manhole in the neighbours garden where the sewer turns but there's no evidence of this when i peek over the wall, the neighbour is quite elderly and i'm not sure that she could help in identifying where this manhole is or even if it exists.
The sewer is shown as 150 unk which i believe means 150 diameter unknow of what the pipe is made from. Going by the plan it looks to be approx 1000-1500 inside her boundary and my planned extension would be built approximately 400mm from the boundary which is a 1800 high brick wall, essentially i believe that the sewer is within 3m, my question is firstly does the boundary wall have any bearing on whether my extension is within 3m, secondly if it doesn't which i'm tending to feel it wont, how do i proceed. Do i get a drain survey and if so can it accurately map the sewer run and depth or do i need to ask permission to dig my neighbours garden to manually find the drain. Or am i getting this in the wrong order and maybe i should get bldg regs drawings first?
The attached pic shows my intended extension in red.
Any help would be appreciated.
Matt

 

Doesn't you're brickie relative know about foundations?

All you need to do is make sure your foundation depth is deep enough so as to not put any load on the drain.

So normal 1m deep may be ok unless the drain is a lot deeper.

You may be able to self certify. Check your water company's guide

 

Thanks for the reply Woody, my brickie knows a bit about foundations but not a lot about public sewers and buildover/self certifying.
I am in Southern water catchment and they say i can self certify if i show the exact location, depth and condition of the sewer and providing that i dont build within 500mm and the footing should extend 150mm below the sewer. I guess i was hoping that someone would either say that the boundary wall would provide a barrier and rule out the need for determining the location of the sewer, or maybe confirm how to determine the exact location of the sewer without digging a hole.

 

The boundary makes no difference.

If you have that drain plan, then you go off that. A CCTV survey will map out the drains and depths.

 

Perfect, Thank you.

 

Today i have had a pre build drain inspection in order to confirm the location of the sewer which is shown on the drainage map provided by Southern Water,
It turns out that the sewer shown on their map, running parallel to my proposed extension inside my neighbours boundary is non-existent.
My drainage joins the public sewer approximately 20m from the rear of my property, the sewer then falls to the right for at least 30m more.
I haven't had the report back yet but this is just from speaking with the guy doing the survey, is this normal for the sewer map to be so wrong?

 

Totally normal!

 

Happens fairly regularly.

 

You two agreeing?! It’s a Christmas miracle

 

I'm southern water too. When BC wrote to them about the drains they came back with a different set of rules compared to the ones stated on their website. I built over the drain with BC happy and without a build over agreement! Foundations had to be 150mm below drain invert and 500mm away... Could only build over clay pipes. No internal manholes. Saved me £800...

 

BC?

 

Building Control

 

Building Control wrote to Southern Water? What weird alternate reality is this?

 

Building Control do normally write to water companies and then applicants receive letters about Build Over Agreements. Water companies were one of our statutory consultee’s for all types of BR applications.

 

Sect 18 if the building act makes it a statutory duty to consult with the water authority if a proposal is over a sewer, however not only are some sewers plotted in the wrong position but it wasn't unusual for them not to plotted at all in one district I worked.