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Earthworks: cut and fill for residential sites

Cut and fill earthworks for residential sites: NCC 2022 batter slopes, controlled vs uncontrolled fill, AS 3798, site classification, retaining wall thresholds.

By Dan Waring. Last updated 8 May 2026. Verified 8 May 2026.

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TL;DR

Earthworks cut and fill reshapes a site to the design pad level before footings go in. The NCC 2022 Housing Provisions Part 3.2 limits unretained embankments to 2 m height and prescribes maximum batter slopes by soil class (e.g. 1:2 for firm clay compacted fill). Any fill placed under a slab or footing must be controlled fill, placed and compacted under Level 1 supervision per AS 3798:2007 and reported by a geotechnical engineer: uncontrolled fill automatically pushes the site to Class P under AS 2870:2011, requiring full engineering design and adding $15,000 to $40,000+ to the foundation cost. Retaining walls kick in when batters exceed the table limits or height passes 1 m: in NSW anything over 600 mm leaves exempt development; in QLD the threshold is 1 m. The earthworks scope must be declared to the geotech so the site classification is correct before the slab design is locked in.

When you do this

Earthworks run between site establishment (fencing, services locates, site office) and footings or slab. On flat or gently sloping suburban blocks, cut and fill may be minor: strip topsoil, trim to pad level, done in a day. On sloping or split-level sites, earthworks can be a separate subcontract, a separate DA condition, and a multi-week critical path item before any structural work begins.

The geotech must be engaged before earthworks commence, not after. If new fill is placed before the geotechnical investigation, the soil report cannot reliably classify the site and the classification defaults to Class P.

Who’s involved

RoleResponsibility
Geotechnical engineerSoil investigation, site classification, specify fill material and compaction requirements, review earthworks plan
Earthmoving contractorExcavation, fill placement and compaction, batter shaping, drainage
Structural engineerFoundation design based on geotech classification, retaining wall design if required
Certifier (private or council)Inspection of earthworks where required by the CC or building permit conditions
Geotechnical Inspection and Testing Authority (GITA)Level 1 supervision of fill placement per AS 3798:2007, issue compliance report
SurveyorSet-out to design pad level, RL (reduced level) checks

Steps

1. Obtain the soil report before planning earthworks

Engage a geotechnical engineer to investigate the site before any earthworks design is finalised. The report classifies the site per AS 2870:2011 (Class A through to P) based on ground movement characteristics. The classification determines what foundation type is permissible and whether any fill placed on site will count as controlled or problematic.

Critical point: if the site has pre-existing fill from a previous development, demolition, or council works, it is almost certainly uncontrolled fill. The site must be classified as Class P until that fill is investigated, characterised, and either removed or formally reclassified under AS 3798 supervision.

Source: AS 2870:2011 Residential Slabs and Footings, Standards Australia (verified 2026-05-08).

2. Understand the cut and fill limits under NCC 2022

The NCC 2022 ABCB Housing Provisions Part 3.2 sets out the deemed-to-comply rules for unretained cut and fill embankments on residential sites. Key limits:

Fill (unretained embankment):

  • Maximum height: 2 m above natural ground level at any point
  • Must be placed and mechanically compacted in layers no more than 150 mm
  • Must comply with the batter slopes in Table 3.2.1 (see below)
  • Where the fill supports footings or a slab, must comply with Housing Provisions Part 4.2

Cut (unretained embankment):

  • Must remain within the allotment boundary
  • Must not undermine the zone of influence of existing structures
  • Must comply with the batter slopes in Table 3.2.1

Any earthworks outside these limits require a retaining wall or engineered solution; the standard deemed-to-comply path no longer applies.

Source: NCC 2022 ABCB Housing Provisions Part 3.2 Earthworks, ABCB (verified 2026-05-08).

3. Apply the correct batter slopes

Table 3.2.1 in NCC 2022 Housing Provisions Part 3.2 specifies maximum embankment slope ratios (H:L, height to horizontal length) for cut and compacted fill:

Soil classSite cut (H:L)Compacted fill (H:L)
Stable rock (Class A)8:13:1
Sand (Class A)1:21:2
Firm clay (Class M to E)1:11:2
Soft clay (Class M to E)2:3Not suitable

Reading the table: A 1:2 ratio means 1 m of rise for every 2 m of horizontal run (a gentle slope). A 1:1 ratio is a 45-degree slope. Stable rock allows a much steeper cut face.

Where soft clay is present, unretained compacted fill is not permitted: a retaining wall is required.

Source: NCC 2022 ABCB Housing Provisions Part 3.2 Table 3.2.1, ABCB (verified 2026-05-08).

4. Distinguish controlled from uncontrolled fill

This distinction has direct consequences for the foundation design and cost.

Controlled fill (also called engineered fill or structural fill): fill placed and compacted under Level 1 supervision per AS 3798:2007, with a Geotechnical Inspection and Testing Authority (GITA) overseeing the work and issuing a compliance report. The report certifies compaction, layer depths, material suitability, and that footings can be placed into the fill. Controlled fill allows the site to retain its underlying soil classification for foundation design purposes.

Uncontrolled fill: fill placed without AS 3798 supervision, or pre-existing fill with no compliance documentation. Under AS 2870:2011, a site with uncontrolled fill cannot use the standard deemed-to-comply slab and footing tables: the site classification becomes Class P and a full engineering design is required.

The AS 3798:2007 Level 1 program requires:

  • Geotechnical engineer (or GITA) on site during fill placement
  • Visual inspection of material suitability (no organics, debris, oversized rock in top 600 mm)
  • Compaction testing at specified frequencies (typically nuclear density gauge or sand replacement tests)
  • Layer-by-layer sign-off: layers must not exceed 150 mm for clay fill, 300 mm for sand fill
  • Completion report certifying compliance with the specification

Source: AS 3798:2007 Guidelines on Earthworks for Commercial and Residential Developments, Standards Australia (verified 2026-05-08); AS 2870:2011, Standards Australia (verified 2026-05-08).

5. Understand the Class P site implications

A Class P classification under AS 2870:2011 means the site has problematic conditions, which includes uncontrolled fill, soft soils, poor drainage, or abnormal conditions. Class P sites cannot use the standard prescriptive slab and footing designs from the AS 2870 tables.

Consequences in practice:

  • Full engineering design required (structural engineer, sometimes geotech providing specific foundation recommendations)
  • Slab panels may need to be suspended or deepened beyond standard proportions
  • Additional soil investigation may be needed (further boreholes, dynamic cone tests, laboratory testing)
  • Foundation cost premium is typically $15,000 to $40,000 above a standard Class M stiffened raft design, depending on fill depth and site conditions
  • The site classification must be reassessed any time the subsurface profile changes due to additional cutting or filling

If site cuts reduce fill depths below the threshold specified in AS 2870:2011 clause 6.4.2, it may be possible to avoid suspended panels. This is worth discussing with the geotech before committing to the earthworks design.

Source: AS 2870:2011 cl 6.4.2, Standards Australia (verified 2026-05-08).

6. Know the retaining wall thresholds

When the required cut or fill height exceeds the batter limits in Table 3.2.1, or when the site geometry does not allow room for a batter, a retaining wall is required. The wall must comply with AS 4678:2002 Earth-retaining structures, which covers walls greater than 800 mm and less than 15 m, with a design life of at least 60 years for permanent residential structures.

When you need approval (state thresholds):

Different states set different exempt development height thresholds for retaining walls. These are the planning exemptions, separate from the structural design requirement:

StateExempt development heightNotes
NSWUp to 600 mmUnder SEPP (Exempt and Complying Development Codes) 2008 cl 3.30 (verified 2026-05-08). Over 600 mm: CDC or DA required. Support for earthworks over 600 mm requires a professional engineer’s certification of structural soundness.
QLDUp to 1 mPer Brisbane City Council requirements; over 1 m requires building approval. Wall within 1.5 m of a building or another retaining wall also triggers approval. (verified 2026-05-08)
VICUp to 1 mCouncil approval generally required over 1 m (verify with the relevant council).
WAUp to 500 mmBuilding approval required if wall exceeds 500 mm from natural ground level (verify with local government).

Engineering requirement regardless of height: Any wall supporting surcharge loading (vehicles, structures, adjacent footings), any wall near a boundary, or any wall in problematic soil conditions requires structural engineer design under AS 4678:2002.

Source: AS 4678:2002 Earth-retaining Structures, Standards Australia (verified 2026-05-08); NSW SEPP (Exempt and Complying Development Codes) 2008 (verified 2026-05-08); Brisbane City Council retaining wall guidance (verified 2026-05-08).

7. Tolerances and acceptance for earthworks

Finished pad levels are set by the surveyor against the engineer’s reduced level (RL) specifications. The earthmoving contractor is responsible for achieving the design pad level within the tolerances specified by the structural engineer. Typical tolerances for residential earthworks:

ElementRequirement
Finished pad levelPer engineer’s drawings; typically ±25 mm of design RL
Fill compactionPer geotech’s specification and AS 3798:2007; reported in the Level 1 completion certificate
Batter slopeMust not exceed Table 3.2.1 ratios; verified by site inspection
Retaining wall plumb and levelPer AS 4678:2002 and engineer’s construction tolerances
Slab edge termite exposure zone75 mm minimum slab edge exposed above finished ground (AS 3660.1:2014)

Per the HIA Guide to Materials and Workmanship, the specific workmanship tolerances for finished earthworks pad levels in residential construction are: per current HIA Guide to Materials and Workmanship. Verified numerical value pending HIA member access. [HIA-039]

Documents needed

  • Geotechnical report with site classification and fill specification
  • Earthworks plan showing cut and fill volumes, batter slopes, finished pad RL
  • Construction Certificate or building permit (which may list earthworks inspection hold points)
  • AS 3798 Level 1 completion report from the GITA (mandatory if fill supports footings or slab)
  • Retaining wall engineering drawings and structural certification (where walls are required)
  • Surveyor’s RL check after earthworks completion
  • Any council or certifier sign-off on earthworks where required by the consent conditions

Common holds

  • Geotech not engaged before earthworks. Fill placed without AS 3798 supervision becomes uncontrolled fill, forcing Class P, requiring full engineering design. The cost to fix this is always higher than the cost to do it right the first time.
  • Pre-existing fill not identified. Sites with fill from previous demolition, past development, or council roadworks often have no compaction documentation. This is automatically Class P until proven otherwise. Check DA or title history before assuming clean ground.
  • Batter too steep for soil class. A 1:1 batter cut in soft clay is not permitted under NCC 2022 Housing Provisions Part 3.2. Site geometry review against soil class is required before the earthworks scope is priced.
  • Retaining wall not in contract scope. Builder quotes slab and frame but the site needs a retaining wall. The wall is either a PC item (unknown engineer design cost) or is omitted entirely. Get the wall designed and priced before contracts are executed.
  • Fill material not suitable. Organic material, soft clay, expansive soils, and demolition debris are not suitable as structural fill. The geotech specifies the material. Using reject fill voids the Level 1 certification and triggers Class P.
  • Drainage not integrated. Fill that retains water builds up pore pressure, reduces effective compaction, and undermines retaining walls. Surface and subsurface drainage design must accompany earthworks, not follow it.
  • Site re-cut after classification. If additional cutting or filling happens after the geotech report is issued, the classification must be reassessed. The structural engineer needs to be notified before a slab is designed on a site that has since changed shape.

References

See also

Last updated: 2026-05-08. Verified: 2026-05-08. Quarterly review for currency.

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