Sewer under property and close to water main

16 Aug 2023
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United Kingdom
We are close to exchange on a house constructed in the 1930s and the searches indicated that a 225mm foul sewer is within the property boundary running longitudinally underneath the house. There is another sewer at the end of the garden.

Search also revealed a water main (in blue, size unknown) running down the alleyway adjacent to the house but out of the property boundary. The alleyway is quite thin so I estimate the distance to be around 1/1.5m to the wall of the house.

There is a question mark on whether the house has already been extended to the rear over the sewer. Surveyor thinks that it might be an extension due to different construction materials but he cannot be certain. Sellers soc says there is no build over agreement in place and no indemnity insurance will be provided.

We were hoping to do a small extension to the rear and add an outbuilding/workshop to the end of the garden but this is looking more unlikely.

Reading around this seems like phenomenal amount of risk to undertake:
  1. If there are problems with the sewer and major works are required, could the water board tear down walls to get access at my expense?
  2. Not able to build within 2 or 3 meters of the water main as this will be a blanket rejection to future development
  3. Even if we could get permission to build over the sewer it is extremely close to the property boundary - may not be possible to dig deep enough for foundations as would encroach on alleyway
  4. Future resales issue as any buyers would also encounter the same problem, not being able to extend has a significant impact on the value.
The main house was built in 1930, would they have built the property over a sewer that time? I’m wondering if the sewer map is incorrect and it is actually positioned down the alleyway at the same location as the water main. The man hole position indicated on the map also appears to be incorrect and there doesn't appear to be any manholes on the property. Can a CCTV survey accurately determine if the sewer runs underneath or down the alleyway?

I'm trying to get an understanding of the risks before proceeding with the purchase. If I call Thames water, would they be able to determine the likelihood of future build over acceptance? I understand that this will invalidate any indemnity policy should we seek one so I am reluctant to do this currently. Thanks and any help/suggestions would be appreciated.
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If you look at Thames Water website the criteria for build-overs the criteria is usually pretty clear as to what they will and will not permit, the drainage maps can be notoriously inacurate sometimes by tens of meters. But that map suggests there are manholes in the road in front and to the rear of your property? Drains can be accurately surveyed to find their location, a CCTV and possibly an above ground tracing survey would be wise to find out the size, depth, condition and exact location if you plan on developing the property further. You would have to do that anyway if you were to extend.
It was only in the past 10 years or so that the water boards adopted public sewers, so if the house was extended well before that time, then a build over agreement with Thames wouldn't have been necessary as they'd have not been the owner.

If the house ticks all of the right boxes, it wouldn't put me off. You can get an agreement now for building within 3 meters, I did so and it was with Thames.
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225 comes out at 250 externally, not 200.
I've never seen pipe or fittings for such a diameter pipe, in builders merchants.

Don't know the exacts, can't remember, so let's say it was 250mm then, as it certainly wasn't the 150mm's, the build over pipe was MUCH bigger than the 150mm's leaving the house.
TW will expect you to pay to survey their pipes / drains and tell them where they are

happened to us

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