I have a main Thames Water sewage pipe running through my property, which takes both foul water and surface water. My neighbour's pipe (225mm at present) joins into this main pipe. We need to, for renovation purposes, change the route of this pipe. This process would include replacing the pipe from his side to the main pipe in our house. He is insisting that we replace his 225mm pipes with the same size pipes, however I have been informed by my builder and surveyor that a 4in (110mm) pipe is sufficient to take the full load of his house. We are interlocked in a dispute at the moment, so my question is, what is the legal requirement of a sewage pipe diameter in the UK? Can we install a 4in pipe, or do we have to comply with his demands? Any help would be greatly appreciated, as it seems that all newdevelopments nowadays have 4in pipes, but I have no legal knowledge.

Get the specs for size from building dept/plumbing inspector.

Show to neighbor and inform him that if he wants larger size he will be responsible for additional cost. Do this in writing. Also if u can, include what the added cost would be.

Chuck

which takes both foul water and surface water.

I question the accuracy of the statement. If it is correct that means that the sewage treatment must not only precess the sewage but also the rain water run off. This would not be allowed in US.
If that is not a correct statement, a 4" sewage pipe is more than adequate for a single residence sewage pipe.

Certainly wouldn't know how your legal system works but if I were your neighbor and had legal permission to run my piping through your property and this is your renovation, or a renovation done for your convenience, I too would insist that you replace what's there exactly.

What you can or must do probably depends on legal issues other than approved plumbing practices.