Retaining walls are not dividing fences and are subject to different legislative requirements. When installing retaining walls along boundaries, they must be wholly constructed inside the property – unless you have you consent of the adjoining landowner.
It is a good idea to have the boundary line pegged by a licensed land surveyor to be sure the retaining wall or dividing fence is placed in the correct location.
Retaining walls are only required when a change in level is too much to slope soil between the higher and lower sides, or when site constraints require a level area up to a boundary.
Which neighbour is responsible for installing retaining is sometimes difficult to determine. Often one owner will have lowered their site and the other filled theirs, causing the different heights at the boundary. Talk to your neighbour about how level differences might be resolved. Note that if you are removing the fence to install the retaining wall, you must consult with your neighbour. If you cause damage to the dividing fence, you must ensure it is repaired.
There are a number of suitable materials for the construction of retaining walls, but some are better than others depending on soil type, height of the retaining wall, and if you are on the high or low side.
Limestone retaining walls rely on their mass to provide support. A rule of thumb when considering limestone retaining is that the base of the wall will be at least half of the height and additional backing blocks are required behind the wall. An engineer is required to design a suitable retaining wall as you may need to support a neighbouring driveway or structure, or your own.
Cavity brick retaining walls are do not require backing blocks, but may need a wide footing to provide strength to the wall. The most space efficient choice is concrete panel and post-style retaining walls, however a structural engineer must confirm the suitability and design of retaining walls over 450mm in height, and a building permit is required from the local authority.