extension over thames water drain - built in 1970's

[forevermore]

buying a house with an extension built in 1976. seems permission granted then but currently would not be allowed to buildover a drain that is there if i applied today.

what are implication of this?

you can build over a TW drain, but you have to enter into an agreement with them - it costs about £350. Probably back in the 70s they didn't do build over agreements. There would be no legal implications,and if it is owned by TW, they are responsilbe for its maintenance.

Happened to me - victorian terrace in Suffolk, drain built over in 70s, no build over agreement. Anglian Water said that the BOA didn't exist back then, so no problems not having an agreement in place. This was to do with a house sale, and the purchasers and their solicitor were happy with this explanation.

what if the drain/pipe became blocked or damaged; could they for example, knock down th extension to fix it and not pay expenses for repairs?

just found out the drain which extension is built over is 'private' and our responsiblity but the extension is within 3 metres of a thames water manhole which resides on the neighbours land