The expert team at Structural Retaining Walls know that retaining walls are critical on infill lots because subdivided blocks often have altered ground levels, limited space, and close boundaries. Without properly engineered retaining walls, soil movement, drainage issues, and boundary disputes are common. In Perth, infill retaining walls must manage long-term soil pressure while complying with council and engineering requirements.
What is the biggest mistake homeowners make on infill lots?
The biggest mistake is assuming retaining walls are landscaping rather than structural elements. Many infill retaining walls fail because they are under-designed, poorly drained, or not engineered to support fences, driveways, or neighbouring soil levels.
Do infill retaining walls need engineering?
Yes. Most retaining walls on infill lots require engineering due to limited setbacks, boundary proximity, height differences, and surcharge loads. Even lower walls often need engineering to ensure long-term stability and council compliance.
Why Infill Lots Are Different
Infill lots are created when larger residential blocks are subdivided into smaller parcels of land. While this allows Perth homeowners to build in established suburbs, it also introduces challenges that don’t exist on standard greenfield sites. Unlike new estates, infill blocks often sit between existing homes with established ground levels, fencing, and drainage paths. Space is limited, access is restricted, and level differences between properties are common. Retaining walls are almost always required to make the site buildable and compliant.
Altered Ground Levels Are the Norm
During subdivision, natural ground levels are rarely left untouched. Blocks are cut, filled, or regraded to meet building and access requirements. This means one lot may sit higher or lower than its neighbour, creating the need for retaining. In Perth, the legal responsibility is clear: each property owner must retain their own altered soil. On infill lots, this often means boundary retaining walls are required from the outset. Assuming the neighbouring fence will “do the job” is a costly mistake.
Limited Space Changes Everything
One of the most overlooked challenges with infill retaining walls is space. Traditional retaining wall designs rely on room behind the wall for drainage, footing depth, and backfill. On infill lots, there is often very little room to work with. This lack of space increases the importance of proper engineering. Walls may need deeper footings, stronger materials, or specific drainage solutions to perform safely within tight boundaries. A generic design that works on a large block may fail on an infill site.
Boundary Walls Carry Extra Risk
Most infill retaining walls are built on or very near property boundaries. This increases the risk of movement affecting neighbouring land, fences, or structures. Boundary retaining walls must be designed to manage soil pressure without encroaching on adjacent properties or redirecting water. Even small movements can lead to fence leaning, cracking, or neighbour disputes. In Perth, councils scrutinise boundary retaining walls closely, especially where height differences are significant.
Drainage Is Often an Afterthought
Drainage problems are one of the most common causes of failure on infill lots. Subdivision changes how water moves across land. Roof runoff, paved areas, and reduced soil absorption all increase water pressure behind retaining walls. Without properly designed drainage, water builds up behind the wall over time. This pressure causes slow movement that may not be visible for years. By the time cracking or bulging appears, the wall is often already compromised. Good infill retaining wall design manages water from day one.
Existing Structures Add Load
Infill sites are surrounded by existing homes, garages, and driveways. These structures place additional pressure on retained soil, even if they are not part of the new build. A retaining wall that supports soil beneath an existing driveway or fence must be designed to handle that load. Many homeowners overlook this and build walls designed only for soil pressure, not real-world conditions. This is a major reason infill retaining walls fail years after construction.
Engineering and Compliance Are Not Optional
Because infill lots are constrained and high-risk, engineering is often mandatory even for lower retaining walls. Perth councils regularly require engineering certification under AS 4678 for infill retaining walls, especially near boundaries. Ignoring engineering requirements can lead to approval delays, stop-work notices, or costly rebuilds. A wall that is non-compliant may also affect insurance coverage or future property sales. Engineering ensures the wall performs safely for the long term, not just at handover.
Timing Matters on Infill Projects
Retaining walls should be planned and built early on infill projects. Waiting until after construction begins often creates access problems, higher costs, and design compromises.
Early installation allows retaining walls to be integrated with site works, drainage, and building design. This reduces risk and helps the entire project progress smoothly. On infill lots, sequencing is just as important as design.
Common Infill Retaining Wall Mistakes
Many Perth homeowners assume the developer has already addressed all retaining requirements. Others reuse old retaining walls that were never designed for new loads. Some rely on fences to retain soil or build walls without considering drainage or future structures. These shortcuts often work temporarily but fail over time. Infill retaining walls demand a higher standard of design and construction.