Our solicitor searches have finally come back and have shown that public sewers run through the garden of the property we are currently SSTC on.
There's currently a conservatory built parallel to the garage - we planned to knock this down and build a kitchen extension in its place. I've labelled the planned extension area in green dots on the pic attached. The red outline is our entire plot, and the gold/blue arrows are the two drainage pipes. The garage was built at the same time as the house, but not the conservatory (don't know when this was built).
However, given the drainage issue, I think we'd need to apply for build over permission from the water supplier if the extension would be within 3m of the pipes. I can't tell from the search results if it would come within 3m - the diagram says it's not to scale.
I rang the water company who couldn't tell me anything as I wasn't the homeowner. I then spoke to our solicitor who said there wasn't that much they could do, but could ask the seller to enquire with the water company on our behalf. However, the solicitor noted this might be a long shot given the sellers are under no obligation to, and they/their solicitors haven't been massively helpful in responding to our solicitors so far anyway.
I think our other option might be to take out a build over CCTV survey, but I don't know if we can do this before owning the house? The 2 companies I've spoken to about this have been a bit unclear about whether we can do it before having the keys (don't know if that's a red flag for the companies!).
We wouldn't want to buy the house if we couldn't do the planned changes to the kitchen, so we need to figure out what to do sharpish before we spend any more time and money! Any advice on what we should do would be much appreciated!