Advice on moving a manhole when building an extension over a sewer

[SilentRefluxAdvicePlease]

Hello, I am looking for advice and experience from anyone who may have built an extension to their property and had to consider an inspection point/manhole cover. The sewer that serves our property, and the properties of our neighbours (at least four semi-detached houses) runs behind our home and the extension we would like to build would in effect go over the top. I know we could not have (nor would want) an inspection point inside the extension, so my understanding is that it would need to be moved further along the existing sewer, so that we would effectively have a new inspection point to the side of the extension. Can anyone please comment on the potential costs and work involved here? I am thinking mainly about who we would need to involve and how we should consider the property boundary/foundations of the extension. Thanks so much!

I think you have to contact your local water provider. They approve and inspect and sign off the work.

Thames water have technical guidance that explains the details. From memory the outside off the chamber needs to be 500mm away from foundations. Depth matters too. If too deep distances change.

They also want the replacement pipe and cleaner to match the existing. E.g clay and brick.

Inspection chambers are also where pipes feed into the main flow. You also need to consider whether you want a join under your extension or reroute those pipes too.

We were quoted £10k incl VAT so considering if it's worth it.

you will need what is called a Build Over Agreement from your water company.
That is a legal permission without which the council's building control will not sign off your works (making the extension a no go). That certifies that the loading imposed by the extension will not crush the drain/pipes running under the building.

In addition, since you mention a "manhole", that implies there is a junction of at least 2 drains meeting at that point. As you also imply the drains serves more than just your own house, it will be classed as a "public" drain meaning legal responsibility for it rests with the water company.

As such the water company will require you to reroute the pipe run(s) in order to ensure that a "manhole" (inspection chamber) can be retained "nearby" and thus continue to allow the water company access for maintenance purposes.

Here is some info
What is a Build-Over Agreement? | 2026 Guide | houseUP