This is the footing question behind every house on a slope. The sloping blocks guide covers what drives the cost; this page explains the three options it refers to.
What are the three ways to build a floor on a slope?
- Cut and fill: excavate the high side, fill the low side, retain both, and pour a slab on the level platform. Familiar and often cheapest on gentle slopes.
- Suspended slab: a concrete floor held up on piers or walls, with the ground left closer to natural below it. Common on steeper sites and for garages and decks.
- Piers or pole frame: a framed floor on steel or concrete piers, or on poles, with minimal disturbance of the slope. Often called a pole home.
Many Illawarra houses mix them: a slab on the lowest level and suspended floors above, which is how most split-level homes are built.
How do they compare?
| Cut and fill | Suspended slab | Piers or pole frame | |
|---|---|---|---|
| Excavation | Most | Less | Least |
| Retaining | Most, often engineered | Some | Least |
| Disturbance of unstable ground | Highest | Moderate | Lowest |
| Floor feel | Solid slab | Solid slab | Framed floor |
| Underfloor space | None | Possible storage or garage | Open or enclosed below |
On landslip-prone ground, the geotechnical engineer may limit how deep you can cut, which pushes the design towards suspended floors. See geotechnical reports.
What is the 600 mm earthworks line?
Under the NSW exempt development rules, earthworks and retaining walls more than 600 mm above or below existing ground level are not exempt: they need approval, and the structural support needs engineering. On a sloping block, cut and fill crosses that line quickly, which is one reason the cost of cut and fill climbs faster than owners expect.
How does drainage change between the options?
Cutting into a slope intercepts groundwater, so the high-side retaining wall needs subsoil drainage behind it. All stormwater then has to reach a legal point of discharge. Wollongong City Council prefers sealed pipes to the street gutter where the gradients allow and does not permit charged lines, so on a block that falls away from the street, an easement through a downhill neighbour or an absorption system may be needed. Raised floors disturb less of the natural drainage.
How do you choose?
The geotechnical report and the survey decide most of it; budget and how you want the house to feel decide the rest. Get both before the design is drawn, and have the builder price the footings with them.
For a block on the escarpment, talk to a builder experienced with suspended slabs and stepped footings on escarpment blocks before settling on a plan that assumes a flat platform.
Sources
Checked on 25 October 2026.
- NSW Planning Portal, Earthworks, retaining walls and structural supports: exempt development standards.
- Wollongong City Council, Domestic stormwater drainage systems: sealed pipes to the street gutter where gradients allow, charged lines not permitted, easement and absorption alternatives.
- Australian Building Codes Board, NCC Part 3.2.4 Site classification: the A, S, M, H, E and P classes.
- Wollongong City Council, DCP 2009 chapter list: E12 Geotechnical Assessment of Slope Instability, E14 Stormwater Management, E16 Bushfire Management, E19 Earthworks.
General information from a builder, not engineering or planning advice. Your geotechnical engineer, structural engineer and certifier decide what your site needs.