If the extension has got final certificate, founds, drainage etc should have been inspected. Cert would not have been issued without electrical part P.
If you are unsure about drainage, you could get a cctv done.
The roof is poorly done and looks like its had a coat of roof sealer indicating its been leaking. The pipework could be tidied up and then it could be boxed in so just the vertical 110mm foul is visible and a proper boot fitted (its possible to obtain split boots for retrofitting).
It comes down to whether the vendors will drop by enough to give you enough of a safety margin to be confident you will have enough in hand to cover remedial work.The vendors will have to decide whether they think there is much liklehood of a buyer not seeing the faults and offering near the asking -which I think unlikely.
There is some risk in buying the property, not so much with what you can see but the risk of the unknown elements, like founds, steels, drains -you would need those elements checked.
So could the property have been previously bought with a half finished extension by the vendors who then finished it themselves badly?
Lots of people have bought new builds that look great and have even worse problems.
If you do buy the property, I have a feelibg you might be clocking up a few posts on here