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Is that common to have sewer pipes running through people's gardens?
Yes. Quite common for pairs or whole rows of houses to have foul drains linked together along the back then joining the main sewer.
Your solicitor should do a drainage search which will usually show where the drains run. It’s not always comprehensive though.
Ask your surveyor to tell you what they find. They’ll be able to work it all out by lifting covers on inspection chambers.
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