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1. Hello I am your Expert and thank you for your Question. I am delighted to answer it.
1. You are right to be concerned about the waste pipe running through your property. Essentially, your neighbour will need a wayleave from you to run the piping underground through your property to wherever it discharges. So, you will need to give him permission to do this and to sign legal documents granting the wayleave. There will then be the actual works carried out on your property. So, it depends upon your attitude towards this happening. You can seek some sort of financial recompense for facilitating your neighbour in this regard, or you can simply agree without any money changing hands. However, the decision is yours.
2. There is always the possibilty that if there is damage to the piping in future on your land, then your neighbour will need access to remedy whatever has gone wrong. So, you should impose a clause in the wayleave whereby your neighbour guarantees to repair any problems with the piping in the future should there be any difficulty on your property. So, there must be a covenant to repair in the wayleave.
3. If you require any clarifiction or if there is additional information you wish to share, I will be happy to assist further in the resolution of your question.