A sloping block is often the reason a home ends up with three levels. The lowest level is tucked into the hill and the upper floors step back from the street. The design can work well, but excavation, retaining and drainage then become some of the biggest parts of the build. Here is what needs attention.
Start with the ground
A detail and level survey shows how steep the site really is and where the neighbouring buildings sit. A geotechnical report then tells the engineer what the soil and rock are like, how deep the good founding material is and whether groundwater is likely. On a steep site, these two documents shape the whole design.
Height is taken from the natural ground, so a lower level cut into the slope still counts toward the limit on the downhill side. Our guide on height limits for three storey homes explains how that plays out.
Excavation and protecting neighbours
Deep cuts close to a boundary can undermine a neighbour's footings, fences and paths. Common requirements include:
- a structural engineer's design for any excavation that could affect adjoining buildings
- temporary support, called shoring, where the cut cannot be safely battered back
- an engineer's report for walls near a boundary that adjoin a neighbour's wall
- a dilapidation report recording the condition of neighbouring buildings before work starts
If the house is approved as complying development, the Housing Code limits how deep you can excavate and how high you can fill. Those limits get tighter close to boundaries. A 2024 update allowed engineer-supported pier depths to be excluded from the excavation limit, but the rules on bulk digging still apply. A three storey home usually goes through a development application, so council conditions and the engineering will set the controls instead.
Retaining walls
There are usually two kinds of retaining on a sloping three storey site:
- Walls that are part of the house, such as the back wall of a lower level built against the slope. These are designed by the engineer and inspected with the house.
- Landscape walls that shape the yard, driveway or outdoor areas. These need separate approval and engineering once they pass certain heights or are near a boundary.
Walls built into the hill also need a plan for water. Behind-wall drainage and the right waterproofing on the face stop moisture from building up pressure or seeping into rooms. See our retaining walls service for more on approvals for landscape walls.
Drainage and stormwater
On a slope, water arrives from uphill and has to leave somewhere lawful. The ABCB Housing Provisions set some baseline rules for houses:
- ground next to a slab should generally fall 50 mm over the first metre away from the building
- in most cases the slab needs to finish 150 mm clear of the surrounding ground, with lower heights allowed only in specific conditions
- subsoil drains need a steady fall of at least 1:300 and must discharge to a pit that can be cleaned out
- stormwater must discharge to a point the relevant authority accepts, such as a legal point at the boundary
Lots that fall away from the street raise a particular problem. If stormwater cannot reach the street by gravity, you may need an easement through a downhill neighbour's land or another solution that council accepts. Settle this early, while changes to the layout are still cheap.
Inspections on a sloping build
The mandatory inspection once excavation is done, before footings go in, matters most on a cut site. Stormwater connections must also be inspected before they are covered. Expect your engineer to inspect retaining wall footings, steel and drainage as well. For the full list of stages, see Critical stage inspections: what we check and when.
A plan assessment can test heights, earthworks and drainage early. To discuss your project, visit our three storey homes service or request a quote.
Local help across NSW
CertifiBuild certifies Class 1 and Class 10 work in 39 council areas. Local rules on these council pages are a good place to start: Inner West Council, Central Coast Council, Shellharbour City Council, Bayside Council, Wollongong City Council, Mosman Council.
Suburb pages with local planning notes: Albion Park Rail, Arncliffe, Balgownie, Balmoral, Bardwell Park, Beauty Point, Bellevue Hill, Blakehurst, Petersham, Warnervale. You can also browse every service area.
Sources
- Site drainage rules (Part 3.3), ABCB Housing Provisions, NCC 2022
- Guide to complying development, NSW Planning
- Codes SEPP essential updates 2024 (fact sheet), NSW Planning