Build Over Agreement with Thames Water

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Thames Water wrote to me the other day saying I needed to enter into a build over agreement with them asI am planning a small extension at the rear of my house. Initially, I wrote back saying the sewer is at the front but they said:

Prior to October 2011 the property may have been privately drained. However, as of the above date all shared drainage have become public and Thames Water do not have any record of these sewers. If you are building within 3 metres of any shared drainage a build over agreement will be required. Please send a plan with full drainage layout in order for us to establish if a build over agreement will be required.

Here is a photo of the back of the house:


And here is the proposed work (the maximum depth of the house is to remain unchanged, we're just squaring off the back):


Here is what Thames water sent me when I bought the house this year (I have ringed our house in yellow):


It doesn't look like the sewer we have in our garden is on their plans.

Here's a pic down the manhole cover:


I am guessing we'll need to go ahead and enter into a build over agreement with them. My questions are:

As they have asked for a sewer plan and the sewer isn't on their plan, it'll be up to me to get the plan drawn up. What type of company do I need to engage and what should I be asking for?

Can someone give me a rough guide to how much I should be paying (I'm in London)?

Is there anything I have overlooked?

If anyone has any tips on the best way of proceeding I'd be most grateful. Cheers!
 
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So the drain in your back garden is presumably serving all of the property's and as such is public hence the build over.

Did you do the drawings yourself or employ someone? If you employed someone then they would probably be the first people to call, they only have to draw on the position of the sewer, if neighbouring property's also have a manhole it will be easy to accurately locate the run. If the slabs are 600's then the run looks to be about 2m away from your proposed wall. Generally the foundations will need to be at a depth low enough not to affect the drain so the depth will be important too. Imagine a 45° line up from the drain towards your house, where this imaginary line meets the bottom of the foundation will confirm the depth you need to achieve.

You doing this on a Notice or with Full Plans? If Full plans your designer should have sorted all this out.

It would be worth talking to Building Control before anything else ie actually speaking with the Inspector who deals with your area, not whoever answers the phone, generally they are a helpful bunch and will pop out to see you, sometimes if the circumstances dictate it, they will be happy to deal with it and you won't have to bother with the Build Over.
 
Thank you for the detailed reply. I think I left an important detail out - the drain actually runs under the house, perpendicular to the street. As such I'm fairly certain we need a build over agreement and as Thames Water have asked for a plan I need to get a survey done.

Any help with what type of company I should be engaging and what I should be asking for would be gratefully received...
 
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This thread is slightly confusing.

Does the drain pass under the new extension? Or just under the existing? Your designer or whoever did your drawings should be very capable of doing this, its an hour or so's work. Have you got a designer? Surely you must have unless you did the drawings yourself? If you had drawings done they really should have noted down where the drains are, pretty poor service. Anyway an architectural technician or a surveyor who does planning/building regs stuff is all that is required, explain you need to apply for a Build Over and you need to show the pipe runs, manholes and inverts on a drawing, if they don't know what you're talking about then don't employ them, this is simple stuff.

If it goes under your new extension then Thames will likely want to do a CCTV survey to inspect the condition of the pipe. Sometimes Build Over's go as smooth as silk and sometimes they are a complete nightmare.

So does the pipe run under the new extension or not? If it does not go under the proposed I would still talk to BC first.
 

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