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the buyers of our house have brought this up
Drainage and waste disposal: Approved Document H
If they are that fussed, they can buy insurance to indemnify themselves against any issues caused by possible non-compliance.
Waste pipe work is fairly easy though - route it, cut it, dry fit it, then solvent weld it.
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Depends.
What pipes from the bathroom (bath/shower basin toilet) go in to which kitchen pipe (sink, dishwasher, washing machine) Or is it that the go in to one pipe?
Best to see what the surveyor has said, as they might have added a caveat of not being sure. What type of survey did they get and did you get?
Our bathroom and kitchen grey waste connect into the same pipe before going into the drain - this pipework was there when we bought the house. Is this compliant with building regs/planning as the buyers of our house have brought this up. If not how big of a job is it to fix?