Retaining Walls and Sloped Blocks: What You Need to Plan for in the Southwest

On a sloped Southwest block, retaining isn't an optional extra, it's a fundamental cost item that needs to be understood before you commit to a design.

The Southwest of WA offers some beautiful, sloped building sites, but slope and retaining go hand in hand. Retaining walls are one of the most consistently underestimated cost items in residential construction, and on a challenging site they can significantly affect both the design and the budget before a single room is laid out.

When retaining is required

Any time the natural ground level needs to be modified, whether by cut (removing soil) or fill (adding soil), retaining is likely to be needed. Common triggers include:

•      Slab on sloped ground - the slab sits level while the ground falls away, creating a retained face on the upslope side.

•      Level driveway or garage access - particularly on blocks with a significant fall from street to rear, the driveway may require substantial retaining to both sides.

•      Level outdoor living areas - creating a usable alfresco or garden area on a sloped block almost always requires retaining on one or more sides.

•      Boundary conditions - where the adjacent lot is at a different level, retaining may be required at the boundary to manage drainage and prevent soil movement.

How retaining affects your design

The position and height of retaining walls affects more than just cost. They determine:

•      Where the house can sit - retaining requirements may push the house platform further from a boundary than setbacks alone require.

•      Window outlook - a wall that retains 1200mm of soil on the street boundary changes what your ground-floor rooms look out onto.

•      Drainage - retaining structures collect water, drainage behind the wall must be designed to prevent hydrostatic pressure building up.

•      Fencing and privacy - retaining and fencing are often co-located but require separate design consideration to avoid conflicts.

Frequently Asked Questions

Do I need a building permit for a retaining wall in WA?

In most Southwest councils, retaining walls over 500mm in height require a building permit. Walls over 1000mm close to boundaries may also require engineering certification. Check with your local council before starting construction.

Who is responsible for retaining on a shared boundary?

Under WA's Dividing Fences Act and common law, retaining wall responsibility on shared boundaries depends on which owner is causing the change in ground level. It's not always split 50/50. Disputes over boundary retaining are relatively common, getting clarity before construction starts is strongly recommended.

Can I do retaining works myself as an owner-builder?

Structural retaining works above a certain height typically require a licensed builder and engineering certification. Low-level garden retaining (under 500mm) is generally exempt. Check with your local council for the specific requirements in your area.

Do you know if your site needs retaining? We can help you assess and plan before your design is locked in projects@fastlanedesign.com.au