How Deep Should Footings Be for NSW Builds?

A footing that is too shallow can turn a straightforward build into an expensive repair job. So, how deep should footings be? The practical answer is: deep enough to found the structure on suitable, stable material and meet the engineered design for the site. That depth varies with soil conditions, building loads, slope, existing fill, drainage and the type of structure being built.

For a house, retaining wall, shed, extension or rural structure in NSW, footing depth should never be chosen by guesswork or copied from the job next door. Two blocks in the same street can have very different ground conditions. Getting the excavation right before concrete is placed protects the structure above it.

How deep should footings be on a NSW site?

There is no single depth that suits every footing. Some light, low-load structures on firm natural ground may require relatively shallow footings. A retaining wall, a two-storey addition, a building on reactive clay, or anything founded near a slope may need substantially deeper excavation or a different footing system altogether.

The key requirement is that the base of the footing reaches competent founding material. In plain terms, this means soil that can reliably carry the load without excessive movement, settlement or erosion. Loose topsoil, organic material, uncontrolled fill and soft wet ground are not suitable footing bases.

For residential work, the footing design is commonly based on a soil classification and structural engineering details. Australian Standard AS 2870 is widely used for residential slabs and footings, while the National Construction Code and local approval conditions also apply. Your engineer or certifier may specify footing widths, depths, reinforcement, concrete strength and any required piers or deepened beams.

Depth is only one part of the equation. A narrow footing that is deep enough can still be inadequate if it does not have the required width, reinforcement or bearing capacity.

What determines footing depth?

Soil type and site classification

Soil is usually the biggest factor. Sand, gravel, rock, clay and fill behave very differently under load and when moisture changes. In the Shoalhaven and Illawarra, sites can range from firm coastal sands and rock to reactive clays, weathered material and steep ground with variable layers.

Reactive clay is a common reason footing designs become deeper or more substantial. Clay can shrink during dry periods and swell when wet, creating ground movement that places stress on slabs, brickwork and walls. The more reactive the soil, the more carefully the footing system needs to manage movement.

A soil test identifies the site classification and helps the engineer design for actual conditions rather than assumptions. This is particularly worthwhile where a block has a history of movement, visible cracking, poor drainage, cut and fill, or nearby mature trees.

Natural ground versus fill

Newly placed fill may look compacted and level, but that does not automatically make it suitable for footings. Unless fill has been engineered, tested and documented, footings often need to extend through it to natural ground or be supported by an engineered solution.

This is one of the most common issues on sloping blocks and newly levelled house sites. A cut-and-fill platform can create different founding conditions across one building footprint. One side may sit on natural ground while the other is over fill, so the footing design must account for the difference.

Building load and structure type

A small garden wall does not carry the same load as a dwelling, masonry retaining wall or commercial shed. As the load increases, footings may need to be wider, deeper, reinforced differently or supported on piers.

Loads are not limited to the weight of walls and roofs. Retaining walls also carry soil pressure, water pressure and surcharge loads from driveways, pools, buildings, vehicles or sloping land above. A footing that works for a freestanding landscape edge is not a suitable design reference for a structural retaining wall.

Slope, water and erosion

Footings near a slope need particular attention. Excavating too close to the edge of a batter can undermine the ground, while surface water running towards the footing can soften soil and cause scour. Drainage needs to be considered alongside the footing design, not added as an afterthought.

Water is also a concern on flat sites with poor drainage. If the excavation base is soft, muddy or disturbed after rain, it may need to be cleaned out, dried, stabilised or reassessed before concrete is poured. Placing concrete onto unsuitable material simply locks the problem in place.

Trees, services and nearby structures

Large trees can affect soil moisture, especially in clay areas. Tree roots and seasonal moisture changes may influence the depth and type of footing required. Removing a mature tree can also change the moisture balance over time, which is why the site assessment should consider both existing and planned landscaping.

Underground services, existing buildings, boundary fences and neighbouring footings can limit where and how excavation is carried out. On tight sites, the sequence of work matters. A practical excavation plan reduces the risk of damaging services or destabilising adjoining ground.

Common footing applications

For a new home or extension, footings are generally designed as part of the slab and structural package. The excavation may include edge beams, internal beams, pad footings or bored piers, depending on the engineering design and soil classification.

Shed footings can appear simple, but the right depth still depends on wind loads, portal frame loads, slab design and ground conditions. Larger rural sheds and workshops often impose concentrated loads at column locations, requiring properly sized pad footings or piers rather than a basic slab edge.

Retaining walls need a purpose-designed footing and drainage system. Their foundations must resist sliding, overturning and bearing pressure, while the wall itself must manage water behind it. If water cannot escape, hydrostatic pressure can build quickly and cause failure even where the concrete work looks substantial.

For decks, verandahs and lightweight structures, bored piers are often used to transfer loads below the topsoil and variable surface layers. The required pier depth and diameter should be based on the design, site conditions and wind exposure, not a rule of thumb.

Why shallow footings cause problems

A shallow footing may initially look fine, particularly in dry weather. Problems often show up later as soil moisture changes, water finds its way under the building, or the structure takes its full load. Signs can include cracked brickwork, uneven floors, sticking doors, wall movement and separated joints.

Repairing failed footings is disruptive. It can involve underpinning, drainage correction, demolition of finished surfaces and structural rectification. Spending the time to investigate the ground and excavate to the specified level is the more cost-effective approach.

The opposite is also true: digging deeper than required without a plan is not automatically better. It increases excavation, spoil removal, concrete volumes and cost. On some sites, deeper excavation can encounter softer material, groundwater or unstable ground that changes the approach entirely. The aim is not maximum depth. It is the correct footing depth for the load and site.

Before concrete is poured

The footing trench or pier hole should be checked against the approved plans before steel and concrete go in. The base needs to be clean, level where required, free of loose material and founded on the material nominated by the engineer. Any unexpected rock, soft pockets, seepage, old rubbish, services or fill should be raised before the pour.

This is where experienced excavation makes a real difference. Accurate digging avoids over-excavation, keeps trenches safe and provides a clean base for the concreting crew. Coffey Civil works across site preparation, excavation and concrete works, helping clients coordinate these stages without handing responsibility between multiple contractors.

If ground conditions differ from what was expected, stop and get direction from the engineer or certifier. A quick decision before concrete is placed is far easier than trying to correct a footing once the structure is built.

A well-built footing is mostly invisible when the project is finished, but it carries everything that matters above it. Start with the site, follow the engineering, manage water properly and make sure the excavation reaches sound ground. That is the practical standard for a footing built to last.