Manhole where extension will be

Either get drainage / grounworker ! .Failing that you can either put sealed man hole in same position, not a great fan of that .Or if they man hole is literally on the main run drain excarvate repair put manhole outside exstention down not up from last position .

As per the regs, sewers (shared Manhole with neighbours) has to be outside of the buildings. Private manhole can be kept inside and it has to be airtight. Neighbours can flush and you can see if their water flows through yours. Drain dye will help in this. For further local guidance find links below. https://www.torfaen.gov.uk/en/Related-Documents/Building-Control/BuildingControl-Regulations/Advice-and-Guidance/Welsh-Water-Building-an-extension.pdf http://secure.thameswater.co.uk/cps/rde/xbcr/corp/building-over-a-public-sewer.pdf https://developerservices.southernwater.co.uk/BuildOver/ApplicationForm

It's usually prudent to relocate the inspection chamber to the exterior of a building. It would be very unfortunate to have a blockage that say couldn't be cleared; that was within a building, and probably covered, possibly with a nice floor.

First check if the manhole is a main sewer or is a private manhole, you can contact Welsh water to see or if you lift the drain lid the man hole will be about a metre deep if its a main sewer but if its private it will be fairly shallow. If its a main sewer you will need to get a contractor who is registered to cut the sewer line and move it, if its private the builder or someone who knows what to do can move it as then it's your responsibility not Welsh water hope this helps

It depends on if the drain is part of a train of houses that use the same flow. If so you will have to use a double sealed drainage inside the extension.