Drain Sewer

[handdog]

Posting on here to see if anyone can help with what to do.
Sister is in a house which was bought 28 years ago.
it has an extension on the kitchen which was there when the house was bought.
Then discovered recently that not only is there a drain in the back garden, but also a main sewer drain running under the extension which must of been built ontop of it. Only discovered this as the sewer drain was blocked and the men came to check it out with a camera,and using that they said there is a drain under the extension, which the floor is concreted over all those years ago.
They did not have the drain on any of their maps showing sewage drains.
Two points here.
So who is responsible?
After 28 years of not knowing and now they want access to come in and dig up the floor and put a drain back in.
Also sister is looking at selling the house and this is of great concern. She has no extra money for investigating and dont know what to do?
Does anyone have any ideas?

Check with local council to see if the extension was granted planning permission, also look through her original documents when she bought the property there might be something referring to the extension.

Is it actually a main sewer or a lateral connection to a main? Who is responsible depends how many properties the sewer serves. If it's only from her property then it would be her responsibility to repair it. It if it serves other properties as well then it is the responsibility of the water company. If it's running across her property rather than away from it then the chances are it's the later.
There are ways of repair or replacing drains without digging up floors which might be an option.

This might help www.water.org.uk/technical-guidance/developers-services/build-over/

Thank you @emmathedilemma that is exactly what my solicitor told me when we got the drains search back. She endeared if my neighbours, who go in to the same drain and had just built an extension, had applied for a builder over. However i note the document doesn't say what happens should there be an issue with the drains under the new build!

The drain serves other properties, runs through from next door into her garden where there is a grate it then curves round to to next drain which is under her property. It's not in need of repair, the problem being it backs up in this area and has to be cleared. They have put camera's through on 2 occasions. They wanted to dig up the floor and place a trap door to which she's refused, they are presently going to leave as it is. And when it occurs again will clear again.