Building an extension near a public sewer - has anyone any experience of this?

[SeriesofUnfortunateEvents]

We are in the process of buying a house. One of the searches revealed a public sewer in the garden of the next door house, right next to the fence of the garden of the house we’re buying. We’re going to want to extend into our garden, small enough that we won’t need planning permission. We’ve been told we’ll need to get Thames Water to approve any work at the back of the house as it would be less than 3 metres from the public sewer. We’re told there would be a legal process and that the result will be a legal agreement which grants Thames Water access rights and other rights. Does anyone have experience of the process - how long it takes and the cost? Also should we be concerned that the rights given to Thames Water would put off potential buyers if/when we sell the house in the future? Thanks in advance for any information.

Yes, we have built within 3m of a public sewer (our extension basically only ends because the sewer is there and we didn't want to actually build over it). But we didn't need a build over agreement because we complied with Anglian Water's (in our case) requirements for not needing one:

1. Sewer less than 3 metres deep.
2. Sewer 150mm diameter or less.
3. Length of sewer under a building must not exceed 6 metres.
4. No manholes or access point permitted under/in a building.
5. Gravity sewer and not pumped.
6. Suitable foundation design to ensure that no loads are imposed on the public sewer.

Building control said a couple of times off handedly that we needed one, but our architect was insistent that we didn't and has shown them evidence (which they have now agreed with) that we didn't need one.

So, are you SURE you definitely need one? If you do, I don't think they're a huge deal to be honest - you can find out most of the information on their website.

PutTheBass thanks very much for such a detailed response. I will check but we are with Thames Water and on their website it seems to be quite clear that you need to get permission if you're building within 3 metres of the sewer which I'm pretty sure would be the case. Thanks again

We did for Thames water and they require an application for any even the simple cases. Ours cost 299 pounds and the architect did the drawing. Basically showing the foundation design relative to the sewer.
The water company already had right of access, so they didn't gain anything more, but there are much more strict requirements for main sewers eg 250mm+ diameter which mean they have to be able to replace the whole thing if needed

Thanks very much johnd2. It never fails to amaze me how generous people on Mumsnet are with their advice.

We bought a house with an unauthorised extension in exactly the circumstances you describe - it's under the neighbour's garden.

Our solicitor arranged an indemnity (paid for by the previous owners), to cover the cost of it ever being an issue. His explanation to us was that if our water company had granted permission for them to build, they would have been obliged to put right any damage to our property as a result.

From the plans, we're compliant with the requirements above anyway (though Severn Trent may have different rules).

Thank you thecats.