On doing some work on my terraced cottage I have found out that the waste pipe to my neighbours toilet in their ensuite has come through the party wall and is now only the depth of the plaster on my side of the house, could you tell me if this is allowed. Have not spoken to the owners yet as this is a holiday cottage and I have not seen the owners yet.

You will have to check with building dept in your area. If the party wall is a fire coded wall, this may be illegal.

It is not uncommon for waste lines to be run in common areas that are not fire walls.

Chuck