We have retaining walls on both sides of our property. Our house was built in the 70's and the neighbour's house in question was built in the 80's. The original retaining wall was built in the 70's and the land next door remained vacant until the current house was built in the 80's and the original retaining wall was added to height-wise and land was filled in on the neighbour's side (no cut on our side) to support the neighbour's new structure. The retaining wall is leaning over toward our house and moving. With the additional height, it is over 1m tall and will require a report from an engineer. My question is, who is responsible to pay for replacing the retaining wall under those circumstances?