Some kind of pump arrangement may be the best option. Excavations for drains, at 3m deep isn't going to cheap and thats if you know where you're digging.
I hate to be the bearer of bad news but if the extension is within 3metres of the sewer you should’ve had a build over agreement….hopefully it’s only the sewer at the rear that is 3 metres down not the side one which closer.
And water authorities often refuse extensions where drains are below 3 metres or bigger than 300mm
You might want to do another thread in the planning and building regs forum, there are architects, structural engineers and surveyors who often answer questions.
Tread very carefully with whoever owns the sewer, they have the power to have extensions demolished.
you won’t be able to sell your house as is.
Id be tempted to find a qualified building surveyor and get an inspection done.
Thanks for all the replies. Bit gutted actually but not surprised. The builder was experienced and he couldn't find it after 3 weeks of looking for it (although we did have an outside grid that he smashed and concreted over so not a lot of sympathy!). I don't think there is a problem with building regs etc as the main sewer is more than 3m away from the property. I think to cut a long thread short... I was really asking if it's possible from a camera survey to identify the general direction i.e. route A B or C in the earlier diagram.
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