Building extensions over drains


We are looking to extend our property in Cranston Road which has an external drain manhole cover in the back garden around a metre out from the back door.

Before we begin the process of speaking to builders etc., we just wanted to see if anyone locally had built over the manhole cover/had their drain moved or had any issues? Would be super helpful to find out what you’ve done if you have been in this situation yourselves so we are fully prepared.

Thank you so much in advance for any help.

Hello. We didn’t build over a drain, but over a sewer (which I think is similar). We had to apply for a Thames Water build over agreement.

Hope this helps. Give them a call. They were very helpful.

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The manhole must be there for a reason. You may find this useful.


We built an extension over a garden manhole cover some years ago, and it posed no problems, though it was not a main Thames Water sewer, so did not require permission from them. As I remember it the building inspector simply wanted us to maintain access to the manhole, which our builder did by cutting a removable access point in the floor in what was previously a bit of our garden. We never had to access it after doing it.


We built an extension over the sewer line running about 3 meters behind our back door. Issues were:

You have to apply to Thames water for permission with drawings etc showing how you will prevent any additional weight being placed onto the sewer, and pay a fee. It takes time so get it done before setting a build date.

You need a manhole where your waste pipe enters the sewer, so that you can clear blockages. So you cant just get rid of one that is in the way

You cant have an internal manhole, so you need to move it if it is in the way. This means backfilling and then digging/building another one further out, and re routing the sewer and your waste pipe to it. Not the biggest job but adds to costs obviously.

If it affects your neighbours you will need to speak with them, not least because they wont be able to use the loo while its being re-routed.

Any foundations that sit across the sewer have to be as deep as the sewer - so in our case the sewer was a full 2 m down, and we have to have 2m foundations, even though this was well in excess of what was needed for the building itself. This is to stop weight being placed onto the sewer.

You need to bridge the sewer line with load bearing joist.

There are minimum distances between the building itself and any manhole, that said, Thames water were reasonably flexible around this.

In the end to save money and hassle we designed the extension to step around the existing manhole, which worked fine for us - so it is now directly outside the back door.

Thna,mes water have a guide on their website to what you ned to consider in the design and have to put in your application form.

Ive attached the drawings we sent to Thames water if it helps


Thank you so much for such a detailed response and your drawings Ems. Its very kind of you. I really appreciate it!

Thanks everyone - really helpful responses. Much appreciated. I’ll let you know how we get on.


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