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The proper way of doing this is to obtain a build-over agreement from the local water authority. The architect or structural engineer working on the extension has to submit drawings of the alternations to the drain layout which then get signed off before works can start.
Our extension would have been on top of a manhole. It was sealed up and a new one put in the garden to give rodding and maintenance access to the drain.
Thames Water will provide an “asset location search” which shows the drains and supply etc (though ours wasn’t accurate at all). Your local authority should be able to provide the same, along with an answer as to whether a build over agreement was issued.
If there’s access from both sides, ie manholes in both neighbours gardens, and at the front, it’ll probably be fine. If they just sealed the manhole up it’s very unlikely anything nasty would make it through the concrete slab floor of a modern building.
See what the water authority say, but I’d use it as a bargaining chip rather than a deal breaker.
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