If the wall is more than one metre high and close to a road or footpath, or more than two metres high elsewhere, planning approval is necessary. Independent, freestanding retaining walls may not need to be approved by building officials. Any construction, however, must be structurally solid and carefully maintained. The soil should be tested for stability before work begins. Retaining walls can be used to create a new look in your yard or to protect existing structures such as fences or trees. They can also be used to prevent erosion of low-lying land or to provide support for elevated buildings. The type of wall used depends on its purpose and location within the property. A boundary wall will usually be needed where there are property boundaries or driveways to maintain. An internal wall can be used where space is limited or if you want to divide up a large area into different gardens. An earthwall works best when you want to create a natural looking structure because it blends in with its surroundings. A concrete wall is most suitable if you want to use the ground as a decorative feature rather than just a foundation. It is important to consider how much force the wall will be expected to withstand without collapsing before it is built. This would depend on how high it is, but also on what kind of material it is made from. Concrete and stone retain walls that are at least 2 meters high and wider than 1 meter require formal approval from building authorities.