build over agreement - Advice

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Hi,

I'm just about to submit some plans for a single rear extension. It has a depth of 3.6 m. However I spoke to Thames water today and my sewer (a class 2) runs parallel to the house. The distance from our house is aprox 3m but the depth is not known. I have no manhole on my property which is good! This means I am extremely close to the drain and do require a build over agreement

Could someone give me an idea of what building work is involved to get foundations laid? The length of my extension is 6.4m.

If anyone has an idea on costs that would be ideal!

Many thanks
 
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If I understand correctly you have a public sewer about 3m off of the back of the house and you want to extend 3.6m which means the sewer will be under the proposed extension.

If so then you need to apply to Thames for a build over approval. For that you will need to accurately plot the exact position of the sewer on a plan and also the outline of the extension. You also need to mark the exact depth. That is submitted to Thames for approval with a cctv survey of the sewer - the submission fee is £225. So you will need to find a manhole somewhere.

Building over is easy as long as you are not right on the corner. You excavate to just below the depth of the drain invert and where the drain runs through the walls you lintel over with 65x100 lintels leaving 75mm clearance either side.
 
thank you for this! I've spoken to them and they say its a class 2 drain meaning £687. They did say they would need to CCTV the drain before and after.

The sewer will run under the extension. I don't have a manhole its in the neighbours garden. When I spoke to them they could not tell me the depth but building control state foundations need to be 1m

Its obvious on the sides that you would put a lintel over them. What I can figure out is how you handle the foundations along the length. These will be aprox 0.3m from the start of the rear foundation.
 
Apologies, I assumed it was 100 to 150mm. Still sounds expensive. A class ii in my area (Wessex) would be £300. I’d also find out how many properties are on the line. Wessex won’t normally allow a build over on a class ii if it serves more than 25 properties. Ask Thames for a map. they should send you one by email if you ask. The maps normally show the depths of manholes so take the nearest one up line and the nearest down line should give you a clue as to the depth. The 1m depth BC quoted is just the minimum for clay soil. If your drain is 1.5m deep you will still need to go just below the invert. The longitudinal foundation can't run along the line of the sewer - it will have to be clear. If necessary you will have to make your extension slightly deeper.
 
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Jeds, you call that expensive. South West Water wanted £1,500 for a 4" sewer. Then expected me to pay for replacing the knackered pitch fibre under my extension. It took a year of argument before the CEO agreed to pay for the replacement pipe after I proved that several of his employees had lied to me several times. So for the OP's sake, I hope Thames water are more helpful, although I would be concerned whether they will allow build over for such a large pipe.
 
Jeds, you call that expensive. South West Water wanted £1,500 for a 4" sewer. Then expected me to pay for replacing the knackered pitch fibre under my extension. It took a year of argument before the CEO agreed to pay for the replacement pipe after I proved that several of his employees had lied to me several times. So for the OP's sake, I hope Thames water are more helpful, although I would be concerned whether they will allow build over for such a large pipe.
I think your thinking of something else. SW Water build over application fee for a 4" sewer is £126.
 
Its obvious on the sides that you would put a lintel over them. What I can figure out is how you handle the foundations along the length. These will be aprox 0.3m from the start of the rear foundation.
As Jeds said you either consider re-positioning the outer wall or you build a complex bridging system via steel beams and pads.

Viability will depend largely upon the depth of the drain. This you need to establish immediately.
 
Jeds, that price only applies if the sewer is good enough to build over, if its knackered they want £1500 up front before they will even consider an alternative.
 
Jeds, that price only applies if the sewer is good enough to build over, if its knackered they want £1500 up front before they will even consider an alternative.
We're not talking about replacing a knackered drain. Just the cost of submitting a simple build over agreement.
 
We do not know if it is a simple build over agreement. That's what mine was until they checked the sewer, then they discovered that it was not fit to be built over so tried to push the cost on me, and wanted £1500 just to start a quote. The sewer in question is class 2, which is a fairly major pipe, and the water authority will want to be certain that it is good enough condition to last the life of the building, or at minimum, that it will be repairable by no-dig methods. The OP wants to be certain that he will not be hit for costs of diverting the sewer if it fails in the relatively near future.
 
Odd development! Dug out old plans including bill from thames water who charged for a class 1 sewer on previous extension. Also puts the sewer in a different position to them.

I spoke to them saying will i be refunded if its a class 1. Answer no! We would have paid cctv costs. Need to measure pipe in next door manhole now.
 
Need to measure pipe in next door manhole now.
This should have been one of the FIRST things you needed to do when considering an extension.

It can make a massive difference to the build, both in cost and practicality.

Poxy architects!
 

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