Minimum Footing Depths for Residential Buildings in Australia

What This Requirement Covers

Minimum footing depth requirements set the vertical position and dimensions of footings, pads, piers and stumps for residential buildings so foundations safely transfer building loads to the ground and limit damage from soil movement, frost or surface deterioration. These requirements exist to ensure adequate bearing capacity, avoid excessive settlement or uplift, and to reduce the risk of moisture-related ground movement impacting the structure. They apply to designers, builders, certifiers and homeowners involved with new dwellings, ancillary Class 10 structures and alterations to existing residential foundations across Australia.

In the National Construction Code (NCC) context, the ABCB Housing Provisions (the prescriptive Part 4.2 in the NCC for housing) sets minimum footing dimensions, depths and construction details for common site classes (A, S and some M sites) and for typical construction types (timber-framed clad, masonry, slab-on-ground, stumps and pad footings). Where a site is classified H, E, P or M with deep-seated movement, the Housing Provisions direct designers to AS 2870 or appropriate engineer design because prescriptive minima do not apply. These provisions should be used together with relevant Australian Standards such as AS 2870 (Residential slabs and footings), AS 1684 (timber framed construction) and other structural standards when designing foundations.

Key Requirements

1. Site classification must be determined in accordance with AS 2870 before using prescriptive footing minima, as required in the ABCB Housing Provisions - see ABCB Housing Provisions Standard 2022, 4.2.2 and Table 4.2.2.
2. Minimum bearing pressure for pad footings - 100 kPa minimum allowable bearing pressure for pad footings is specified in the ABCB Housing Provisions (Table notes to Table 4.2.13g) - see ABCB Housing Provisions Standard 2022, Table 4.2.13g and notes.
3. Pad and square footing dimensions for typical clad frame on Class A and S sites are given as minimum width/diameter and thickness values. See Table 4.2.13g: Minimum dimensions of circular and square pad footings for clad frame, Class A and S sites (ABCB Housing Provisions Standard 2022). Key table notes include:
4. The effective area supported by a pad footing is the sum of supported floor area, supported roof area (if applicable) and half the supported wall area in elevation.
5. Width or diameter can be reduced to one half where footings bear on rock.
6. Footings must be concrete; if excavation is backfilled with tamped soil the footing thickness must be increased by 50 mm (Table 4.2.13g note 6).
7. Strip footing minima for masonry and other systems on Class A, S and some M sites are provided in Tables 4.2.15a/4.2.15b (ABCB Housing Provisions Standard 2022). Notes include:
8. All masonry walls must be supported on strip footings (Table 4.2.15b note 1).
9. Internal strip footings must be of the same proportions as external footings and run from external footing to external footing (note 2).
10. If footings are deeper than the specified depth, reinforcement must be increased accordingly (note 4).
11. Stiffened raft and slab minima for Class A, S and M sites are provided in Tables 4.2.14a-c and Figures in Part 4.2.14; e.g., minimum depth and reinforcement schedules for clad frame on Class A sites (ABCB Housing Provisions Standard 2022, 4.2.14 and tables).
12. Minimum footing depth for stumps and piers: Table entries in Part 4.2 (for example Table 4.2.13g and adjacent figures) specify depth D values for stumps, piers and pads depending on construction type (clad frame, masonry) and site class. Typical examples in the Housing Provisions give D = 300 mm for certain clad frame footings on Class A sites (see Table notes and figure captions in 4.2.13 / 4.2.14).
13. Reference to AS 2870: For sites classified H, E, P or M with deep movement, or where specific conditions (uplift, lateral resistance, expansive soils) exist, the ABCB Housing Provisions require design in accordance with AS 2870 - see ABCB Housing Provisions Standard 2022, 4.2.2 and explanatory notes.
14. Loads and design assumptions: Table notes in the Housing Provisions provide load assumptions (permanent and imposed loads), load combinations and maximum eccentricity allowances; pad footing design must provide an allowable bearing pressure not less than 100 kPa unless engineered otherwise (see Table 4.2.13g notes).

Cited documents and clauses: ABCB Housing Provisions Standard 2022, Part 4.2 (4.2.1, 4.2.2, 4.2.13-4.2.15), Tables 4.2.13g, 4.2.14a-c, 4.2.15a/15b; AS 2870 (site classification, design for reactive soils); AS 1684 where timber framing interfaces with footings; AS 3600/AS 3700 where concrete/masonry detailing is required for engineered footings.

Residential vs Commercial

1. Residential (Class 1 and Class 10) foundations are covered directly by the ABCB Housing Provisions (NCC housing prescriptive Part 4.2) which provides prescriptive minimum footing dimensions and reinforcement for common site classes A, S and some M sites. These tables give straightforward minimum depths, widths and reinforcement for pad footings, strip footings, slabs and stumps for typical housing construction.
2. Commercial and higher-risk buildings (Class 2 to 9) are not covered by the Housing Provisions. Their foundation design must comply with NCC Volume One structural provisions and be engineered to the relevant actions and design standards (including AS 2870 where applicable, and other structural standards such as AS 3600, AS 4100), using site investigations, geotechnical reports and engineered footing depths, sizes and reinforcement. In short, commercial buildings generally require site-specific geotechnical design rather than the prescriptive minima used for ordinary housing.

Exceptions and Exemptions

1. Sites classified H, E, P or M-D (deep-seated movement): prescriptive minima in the Housing Provisions do not apply; design must follow AS 2870 or be engineer-designed (ABCB Housing Provisions Standard 2022, 4.2.2 notes).
2. When footings are founded on rock, certain minimum dimensions may be reduced (for example pad footing diameter or width may be reduced to one half when bearing directly on rock) - see Table 4.2.13g note 2.
3. Where the prescriptive footing is altered (deeper or wider), reinforcement must be increased to match deeper proportions (see Table 4.2.15 notes).
4. For footings providing resistance to uplift or significant horizontal loads (for example piers resisting lateral/wind uplift), designers must refer to AS 2870 and other relevant structural standards; the Housing Provisions note that stumps resisting uplift must comply with AS 2870 (Table 4.2.13g note 7).
5. Alternative detailing or engineered solutions may be used where certified by an appropriately qualified engineer; this is common for unusual loads, poor soils or where site-specific geotechnical recommendations exist.

State and Territory Variations

1. The NCC is a national code, but each state and territory may have schedules or local amendments that modify national provisions. Always check the relevant state schedule in NCC Volume One (Schedules 4-12) and local technical provisions before finalising design.
2. Example notes in the NCC/Housing context: Queensland, New South Wales and other jurisdictions sometimes publish additional technical guidance for footings, minimum depths for frost or scour in coastal zones, or heightened requirements where climatic or soil conditions differ. The ABCB Housing Provisions specifically state that where site classifications or conditions fall outside A, S or the covered M cases, the designer must use AS 2870 and consult the state schedule where local modifications exist.
3. Local councils and certifiers can require geotechnical investigations and engineered footing designs even for residential Class 1 projects in sensitive areas (coastal, reclaimed land, old landfill, steep slopes). Always verify local planning/council and state regulatory schedules.

Practical Compliance Tips

1. Classify the site first - determine site class per AS 2870 before using any prescriptive minima in the ABCB Housing Provisions (see ABCB Housing Provisions Standard 2022, 4.2.2). Misclassification is a common source of noncompliance.
2. Follow table notes exactly - many “minimum depth” values in the Housing Provisions have important table notes (for example the 50 mm increase for tamped backfill, the 100 kPa minimum bearing pressure). Omitting these adjustments leads to under-designed footings.
3. Use engineered design for borderline or unusual sites - if the site is H, E, P or M-D, or if there are signs of slope instability, contamination, previous settlement or fill, get a geotechnical report and an engineer’s footing design rather than relying on prescriptive tables.
4. Confirm reinforcement when changing dimensions - if you increase footing depth or width beyond the table minima, update reinforcement to the proportion specified (Table 4.2.15 notes).
5. Document assumptions for certifiers - include site classification (AS 2870), footing tables used, bearing pressure assumptions (e.g., 100 kPa), and any geotechnical reports in the building paperwork to satisfy the certifier and avoid surprises at inspection.
6. Watch for local amendments - contact the local council or check the relevant state schedule in the NCC for any jurisdictional changes that affect footing depths or design requirements.
7. Address moisture control and drainage - poor surface drainage and garden beds adjacent to footings can cause moisture movement. Maintain drainage away from footings and follow AS 2870 guidance on moisture control to reduce the need for deeper or special footings.

References: ABCB Housing Provisions Standard 2022, Part 4.2 (4.2.1, 4.2.2, 4.2.13-4.2.15), Tables 4.2.13g, 4.2.14a-c, 4.2.15a/15b; AS 2870 (Residential slabs and footings); AS 1684 (timber framing interfacing with foundations); AS 3600/AS 3700 where concrete or masonry structural detailing is required.

Note: This article summarises prescriptive minima from the ABCB Housing Provisions and points to AS 2870 for engineered design. Where documentation does not conclusively address a particular site condition, seek a geotechnical report and an engineer’s design and check any state or territory schedule to the NCC for local variations.