Every year, thousands of new houses across New Zealand are built on a foundation design that references a single number: 300 kilopascals. This figure, drawn from the “good ground” definition in NZS 3604:2011, quietly underpins the vast majority of residential construction consented under a non-specific design pathway. Yet many builders, homeowners, and even some designers misunderstand what verifying “good ground” actually requires — and getting it wrong can mean a foundation that fails building consent, or worse, one that settles unevenly within a decade of occupation.
Understanding the good-ground-nzs-3604 standard isn’t just an academic exercise for geotechnical engineers. It determines whether a site can use the simple, low-cost prescriptive foundation details in NZS 3604, or whether it needs specific engineering design under NZS 3604’s Section 3 exclusions, AS/NZS 1170, and the New Zealand Building Code Clause B1 (Structure). This article unpacks the definition, the verification methods accepted across New Zealand, and the practical steps site owners and designers should take before committing to a foundation type.
What Does “Good Ground” Actually Mean?
NZS 3604:2011 defines good ground as ground that will safely sustain permanent loads with an ultimate bearing capacity of 300 kPa or greater, and which is not left in a loosened state during excavation, is not subject to significant total and differential movement due to seasonal moisture changes, is not on a slope where instability could occur, and is not organic or otherwise degradable soil such as uncompacted topsoil, loose sand, peat, or fill placed without engineering supervision.
This is a compound definition, not just a single bearing pressure figure. A site can technically achieve 300 kPa in a scala penetrometer test but still fail to qualify as good ground if it sits on an unstable slope, is underlain by expansive clay, or includes uncertified fill within the depth of influence of the footings. This is where many site assessments fall short — they test for bearing capacity but neglect the exclusionary criteria that sit alongside it.
Sites that don’t meet all these criteria fall outside the scope of NZS 3604 entirely and require specific engineering design, typically involving a geotechnical investigation report, bearing capacity calculations to NZS 4402 or equivalent methods, and a producer statement (PS1) from a chartered professional engineer. Councils across New Zealand — from Auckland Council to Christchurch City Council — will not issue a building consent for a non-specific foundation design without documented evidence that the good ground threshold has been met.
The Verification Methods Councils Actually Accept
NZS 3604 and the wider body of MBIE guidance recognise several accepted methods for verifying ground bearing capacity, and the appropriate method depends on soil type, site history, and consent authority expectations.
Scala Penetrometer Testing
The dynamic cone penetrometer, commonly called the scala penetrometer, remains the most widely used field test for residential sites. A 9 kg hammer drops 510 mm to drive a cone into the ground, and the number of blows per 100 mm of penetration correlates to an approximate ultimate bearing capacity. This method is inexpensive, fast, and suitable for shallow investigations to around 2–3 metres, but it is unreliable in gravelly soils, where refusal can be mistaken for high bearing capacity, and in soft clays sensitive to moisture content at the time of testing.
Hand Auger and Test Pits
Hand auger boreholes and machine-dug test pits allow direct visual classification of soil type, moisture condition, and the presence of fill, organic material, or groundwater. Test pits are particularly valuable for confirming whether fill has been engineered (compacted in layers under supervision) or simply dumped, since NZS 3604 explicitly excludes uncertified fill from the good ground category regardless of measured bearing capacity.
Cone Penetration Testing (CPT) and SPT
For larger developments, sites with known liquefaction potential, or areas flagged in regional geotechnical databases (such as the Canterbury Geotechnical Database following the 2010–2011 earthquakes), a full CPT or Standard Penetration Test provides continuous, quantitative subsurface data. These tests feed directly into bearing capacity calculations and liquefaction assessments required under MBIE’s guidance for the Canterbury region and increasingly requested elsewhere, including parts of Wellington and Hawke’s Bay following recent seismic activity.
Key Takeaways for Designers and Site Owners
- 300 kPa is a floor, not a guarantee. A site achieving 300 kPa ultimate bearing capacity via scala penetrometer can still fail the good ground test if it includes uncontrolled fill or sits on an unstable slope exceeding the limits in NZS 3604 Section 6.
- Test at footing depth, not just at surface. A common error on sloping Auckland sites is testing near ground level rather than at the actual founding depth of 300–600 mm below finished ground level, where clay content or moisture conditions often differ significantly.
- Document everything. Building consent authorities in Wellington, Hamilton, and Dunedin increasingly require scala penetrometer logs, photographs of test pits, and a written geotechnical statement — not just a verbal assurance from the builder.
- Seasonal timing matters. Testing a clay-rich site in Canterbury during a dry summer can overstate bearing capacity by a significant margin compared with winter saturation. Best practice is to test during, or shortly after, the wettest part of the year, or apply a conservative reduction factor.
- Know your TC zone. In Canterbury’s Technical Category 3 (TC3) areas, and in similarly flagged zones elsewhere, good ground assumptions under NZS 3604 do not apply at all — specific design is mandatory regardless of measured bearing capacity, due to liquefaction risk.
When Specific Design Becomes Unavoidable
Beyond the 300 kPa threshold itself, several common site conditions automatically push a project outside NZS 3604’s scope. These include sites with more than 600 mm of variation in bearing depth across the footprint, sites within identified flood plains or coastal inundation zones under regional plan overlays, sites with retained fill slopes exceeding the standard’s slope stability limits, and any site where reactive or expansive clays are present, since NZS 3604 does not adequately address the differential ground movement these soils generate.
As one Christchurch-based geotechnical engineer with over 20 years of post-earthquake residential experience puts it: “The 300 kPa figure gives people false confidence. I’ve seen sites pass a scala test with flying colours and still fail structurally within five years because nobody checked what was underneath the top 600 millimetres, or whether the fill had ever been compacted properly. Good ground is a whole-of-profile assessment, not a single number on a test sheet.”
This is precisely why MBIE’s own guidance documents, and the commentary accompanying NZS 3604:2011, stress that verification should be undertaken or reviewed by someone competent to interpret the results — not simply performed as a box-ticking exercise before a consent application is lodged.
Practical Steps Before You Build
For any residential or light commercial project in New Zealand, the practical sequence should be: commission a site-specific geotechnical desk study using regional council GIS layers and, where available, the New Zealand Geotechnical Database; carry out field verification (scala penetrometer at minimum, CPT or test pits for anything unusual) at proposed footing depths across multiple points on the site; cross-check results against all five good ground criteria in NZS 3604, not bearing capacity alone; and obtain a written statement from a suitably experienced professional confirming whether the site qualifies for non-specific design or requires specific engineering.
Skipping this sequence to save time or cost rarely pays off. Remedial foundation work after settlement or cracking typically costs many multiples of the original geotechnical investigation, and can trigger disputes with building consent authorities, insurers, and future purchasers during due diligence.
Get Your Site Assessed Properly
Whether you’re a homeowner planning a new build, a developer scoping a subdivision, or a designer needing a defensible foundation recommendation, the question of good ground deserves more than a quick scala test in the driveway. Chambers Consultants provides comprehensive geotechnical assessments, scala penetrometer and CPT investigations, and specific foundation design services across New Zealand, ensuring your project meets both the letter and the intent of NZS 3604 and the New Zealand Building Code. Contact our foundations team today to discuss a site-specific verification plan before your next consent application goes in.