My neighbour recently completed building his house which involved significant fill on our boundary. The council development approval issued for the construction was for both the construction of the house and retaining walls on both sides of the property. My neighbour is proposing to build retaining wall under the fence but is now asking me to pay 100% of the retaining cost as I am on the lower side of the boundary, even though the retaining requirement was mainly caused by the fill to level my neighbours block. Who is responsible for the retaining wall in such circumstances? The council advised even thought the retaining wall is a requirement as per the development consent, they cannot give any legal advise. Appreciate your help.