Two Seismic Design Standards, One Decision: What It Means for Your Project
- 12 hours ago
- 8 min read
Right now, New Zealand has two acceptable ways to design a building or structure for earthquakes, and the choice can affect consent processes, cost, performance and long-term value.
Many building owners, developers and asset managers do not know this choice exists, because it is often made quietly by the design team. It should not be, as it is a decision about how much earthquake performance you are buying, and it deserves to be made deliberately by the people carrying the risk.
In short, New Zealand currently has the long-standing seismic design standard, NZS 1170.5:2004, and the newer technical specification, TS 1170.5:2025. The new specification reflects updated earthquake hazard science, requires the ground response to be measured more directly, and currently follows a less familiar consent pathway.
Which one to use is a real decision about performance, cost, consent risk and future value.
Why there are two standards
New Zealand sits on the boundary of the Pacific and Australian plates, and the last two decades have shown what that means for our built environment. Every new structure must be designed for earthquake loads, and those loads are based on the country’s scientific understanding of seismic hazard.
That understanding changed significantly with the release of the 2022 National Seismic Hazard Model (2022 NSHM). The updated model incorporated newly identified faults, revised understanding of major sources such as the Hikurangi Subduction Zone, advances in engineering seismology and modelling made possible by modern computing.
Figure 1 shows how the 2022 National Seismic Hazard Model changed the estimated peak ground acceleration (PGA) compared against the 2010 model. The maps show the ratio of PGA predicted by NSHM 2022 to PGA predicted by NSHM 2010 for two hazard levels: a 10% probability of exceedance (PoE) in 50 years, roughly equivalent to a 1-in-500-year event, and a 2% PoE in 50 years, roughly equivalent to a 1-in-2500-year event. Areas shown in red indicate where the 2022 model predicts higher shaking than the 2010 model. The darker the red, the greater the increase. The key message is that the updated model generally shows higher seismic hazard across much of New Zealand, although the size of the increase varies significantly by location and hazard level.
For building owners and developers, the important point is not the exact number on the map. It is that the science has fundamentally changed. For many projects, the hazard that engineers are now being asked to consider is higher than the hazard embedded in the older seismic design framework.
These findings created the current transition. NZS 1170.5:2004 reflects the older hazard model. TS 1170.5:2025 reflects the updated science. Both can currently be used, but they point to different levels of shaking and follow different compliance pathways.
What is actually different?
For a non-engineer, three differences matter.
The design loads are generally higher
Because the 2022 model found higher seismic hazard across much of the country, structures designed to TS 1170.5 will generally be designed for stronger shaking. The increase varies significantly by location and site conditions. Some regions change very little, while others change substantially.
The new specification needs the ground measured
TS 1170.5 places greater emphasis on measured ground conditions. A key input is Vs30, the average shear-wave velocity of the top 30 metres of ground.
Vs30 can be measured using non-intrusive geophysical methods such as MASW, or by direct measurement in boreholes or CPT soundings. The new specification also changes how site classes are defined, so how the ground is measured, and what class it falls into, can directly affect the design.
In practical terms, seismic design of buildings and infrastructure under the new specification starts with knowing how your ground will physically respond to seismic waves generated by an earthquake.
The shear wave velocity test methods have been well established for some time, but the requirement for this testing to be undertaken to classify a site is the new part.
The consenting pathway is less familiar
NZS 1170.5:2004 sits within the familiar verification method pathway used by engineers and councils. TS 1170.5:2025 is currently used as an Alternative Solution, which means the engineer must demonstrate Building Code compliance more explicitly.
That is manageable, but it can involve more explanation, more documentation and sometimes peer review. The consent process is likely to get smoother as councils and designers become more familiar with the new specification.
TS 1170.5 also introduces design approaches suited to the updated seismic demands, including provisions relevant to rocking behaviour and non-structural parts and components. These connect to the wider move toward lower-damage seismic design in New Zealand.
How to think about the choice
The honest framing is this: both approaches may be available, but they buy different levels of earthquake performance.
Designing to NZS 1170.5:2004 means designing to a hazard model that is now understood to understate seismic hazard across much of the country. A building designed to it is still designed for life safety under the framework it uses, but in a major earthquake it may sustain more damage than one designed to the newer specification.
That distinction matters. The issue is not only whether people can get out safely. It is also about damage, downtime, repairability, insurability and long-term value.
For a lower-importance structure, a project in a lower-hazard area, or a building governed more by wind than earthquake loads, NZS 1170.5:2004 may still be a sensible and proportionate choice.
For infrastructure owners, government agencies, energy companies and clients whose assets need to keep serving communities after an earthquake, the case for designing to the best current understanding of the hazard is even stronger.
There is also a timing issue. Developments that will be designed, consented and built over several years may benefit from starting on the newer specification, rather than risking a disruptive mid-project transition as the older standard is eventually replaced.
And there is a value question. As the new framework becomes more familiar, buildings designed to the older hazard model may be judged differently by buyers, tenants, insurers and asset managers. Designing to TS 1170.5 is partly about how the building performs in an earthquake, and partly about how it will be viewed over its life.
In terms of the assessment of existing buildings, engineers are to remain consistent with the established seismic assessment framework, referring to the NZSEE Guidelines. As an owner or buyer, you may also request an assessment of an existing building using TS 1170.5:2025 to understand the implications of current seismic hazard knowledge. However, that assessment does not currently form part of the earthquake-prone building legislative framework. The EPB system remains based on the prescribed assessment methodology and seismic risk settings associated with NZS 1170.5:2004.
Regardless of which hazard basis is adopted, the structural and geotechnical engineers should discuss it early, agree how it will be applied across the project, and document the decision clearly.
A side-by-side comparison of the standards
Design aspect | NZS 1170.5:2004 | TS 1170.5:2025 |
Hazard Model | Earlier seismic hazard model | Updated NSHM 2022 |
Seismic Demand | Familiar baseline approach | Generally higher, site-dependent |
Site Classes | 5 site classes (A to E) | 7 site classes (I to VII) |
Primary Basis for Site Classification | Soil profile, depth to rock and material properties | Measured Vs30 and revised site classes |
Compliance Pathway | B1/VM1 verification method | Alternative Solution pathway |
Council Familiarity | High | Increasing |
Often suited to | Lower-importance structures, lower-hazard areas or wind-governed buildings | Major infrastructure, long-life assets, staged developments, higher-hazard areas and resilience-focused projects |
This table is a simplified comparison. The right approach depends on the site, structure, client requirements, consent pathway and project objectives. | ||
Common questions about the two seismic design standards
Can I still use NZS 1170.5:2004 for my project?
Yes, in many cases. NZS 1170.5:2004 remains the most familiar compliance pathway for meeting Building Code requirements.
The trade-off is that it reflects the older seismic hazard model. Whether it is the right choice depends on your project, its location, its importance, its design drivers and the level of earthquake performance you want.
Do I have to use TS 1170.5:2025?
Not yet for most building projects. TS 1170.5:2025 is currently available as an Alternative Solution pathway rather than being the default cited standard.
Some clients and asset owners may require it earlier, especially where resilience and long-term infrastructure performance matter. Requirements are evolving, so the current position should be checked for each project.
Will designing to TS 1170.5 make my building more expensive?
Often, but not always. Because the newer specification generally reflects higher seismic hazard, the structure may cost more. The effect depends on location, ground conditions, building type and whether the design is governed by earthquake or by something else, such as wind.
But cost should be weighed against performance. The newer specification may reduce expected earthquake damage, downtime and long-term risk, which may impact which is more costly in the long run.
There is also an important site-specific point: because TS 1170.5 requires better measurement of the ground, the result may sometimes be less conservative than an assumed site class. Measuring the ground can work in the project’s favour.
What is Vs30, and why does my site need it measured?
Vs30 is the average shear-wave velocity of the top 30 metres of ground. In simple terms, it is a measure of how stiff the ground is.
Ground stiffness affects how earthquake shaking is experienced at the surface. Under TS 1170.5, Vs30 and site class are important inputs into seismic design, so the ground needs to be measured reliably rather than assumed.
That can be done using geophysical methods such as MASW, or by direct measurement in boreholes or CPT soundings.
Will my building consent take longer under TS 1170.5?
It can. Because TS 1170.5 is currently used as an Alternative Solution, the compliance case may need to be explained more explicitly, and some councils may request peer review.
That does not mean it is impractical. It means the engineer needs to understand the specification, the compliance pathway and the evidence council will expect.
What happens next
The two-standard situation is a transition, not a permanent destination. TS 1170.5 is where seismic design practice is heading, and the current technical specification is expected to inform the future replacement of NZS 1170.5:2004.
For owners and developers, the key point is not just what is legal today. It is what level of earthquake performance, consent certainty and long-term value you want to design for.
Where we come in
Cook Costello can help you work through this decision before the design is locked in.
Our geophysicists measure Vs30 and support site classification using MASW, downhole and CPT-based methods. Our geotechnical engineers characterise the ground conditions. Our structural engineers design to the appropriate seismic framework and, where required, help make the Alternative Solution case for TS 1170.5.
The value is having those disciplines in one team. The ground measurement, site classification, structural design and consent strategy can be considered together, rather than treated as separate tasks.
If you are planning a building or infrastructure project and want to understand what the two seismic standards mean for performance, cost, consent and long-term value, talk to us before the design is committed.
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