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What are the five importance levels, from IL1 to IL5?
The five levels are defined by use, occupancy and the consequences of failure. They are set out in Clause A3 of the Building Code and described in more detail in AS/NZS 1170.0. In plain terms:
IL1, low hazard. Structures with a low degree of hazard to life and property. The standard uses a total floor area of under 30m² as a guide, and gives examples like farm buildings, isolated rural structures, towers, fences, masts and in-ground swimming pools.
IL2, normal structures. Everything not placed in another level. Single family homes and car parking buildings are the everyday examples. The large majority of residential and ordinary commercial work sits here.
IL3, structures affecting crowds. Buildings that hold people in crowds, hold contents of high value to the community, or put crowds at risk. The standard is specific: places where more than 300 people can gather in one area, day care over 150 capacity, primary and secondary schools over 250, tertiary and adult education over 500, health care with 50 or more resident patients but no surgery or emergency treatment, larger airport terminals and principal railway stations, correctional facilities, large public assembly buildings, and utilities such as power generation and water treatment that are not designated post-disaster.
IL4, post-disaster facilities. Buildings with a special post-disaster function: the facilities a region needs to keep running after a major event. Medical, surgical and emergency treatment facilities, fire and police stations, emergency vehicle garages, designated emergency shelters and centres, and buildings holding hazardous materials that could affect land beyond the property boundary.
IL5, special structures.Structures that sit outside AS/NZS 1170.0 entirely. Their acceptable probability of failure has to be set by a special study, because failure would be catastrophic across a large area or to very large numbers of people. Major dams and extreme hazard facilities are the examples the standard gives. Note that IL5 is not the “emergency headquarters” tier that some summaries claim: emergency and lifeline facilities are IL4.
How does an importance level change the way a building is designed?
The importance level sets the annual probability of exceedance, in everyday language the return period, for the events a structure is designed to resist. Under AS/NZS 1170.0 Table 3.3, for a building with a 50 year design working life the ultimate limit state design event is roughly a 1-in-100-year event for IL1, 1-in-500 for IL2, 1-in-1000 for IL3, and 1-in-2500 for IL4.
So an IL4 building is designed for an earthquake five times rarer, and considerably stronger, than the one an ordinary IL2 house is designed for. That difference flows through the whole structure: member sizes, connections, bracing, foundations and the robustness detailing that ties everything together. IL4 also carries a second serviceability check so that essential facilities can keep operating, not merely stay standing, after a design-level event. This is where the Building Code Clause B1 Structure performance requirements and the earthquake actions standard NZS 1170.5 come together.
The same scaling applies to wind and snow, not just earthquake, and the same Clause A3 classification also informs fire safety provisions. In practical terms a higher importance level means more structural engineering, a heavier and more resolved structure, tighter documentation, and closer scrutiny from the building consent authority. It is a genuine cost and programme driver, which is why it needs to be settled early rather than discovered late.
Who decides a building's importance level, and how?
The importance level comes from how the building is used and how many people it holds, matched against the categories in AS/NZS 1170.0. For a building type the standard does not list directly, the designer has to exercise judgement, and that is exactly where experience earns its keep.
On my projects the importance level is settled at the very start, because it drives the entire structural brief. I confirm it with the CPEng structural engineer and, where there is any real doubt, with the building consent authority before the design develops. Get it wrong in either direction and it costs: overstate it and you pay for resilience the building does not need; understate it and you face a consent that will not be granted, or worse, a building that is not safe for its actual use.
Mixed-use and staged buildings need particular care, and a change of use later can shift the level. Nailing the classification down and documenting the basis for it is part of a clean consent set, which is what our building consent drawings service is built around.
Why importance level matters if you own, lease or upgrade a building
For existing buildings, importance level is the yardstick a seismic assessment is measured against. A building's percentage of New Building Standard (%NBS) is worked out against the loads required for its importance level, so the same physical structure can rate differently depending on how it is used and classified.
That has real commercial consequences. Tenants who need resilience, government agencies, health providers and some larger corporates, often require an IL3 or higher building and a minimum %NBS before they will sign a lease. It is a common sticking point in commercial property deals. When you alter a building or change its use, the importance level can move with it: turning ordinary space into a place of assembly or a health-care use can push the building up a level and trigger structural upgrade work.
If you are weighing up a purchase, a lease or a change of use, it pays to understand the importance level before you commit. Our commercial building design and seismic upgrade work both start from getting this classification right.
Does Marsco Architecture work on higher importance level buildings?
Yes. Over my career I have worked on buildings across importance levels 1 to 4, including high-consequence environments such as hospitals, laboratories and fire stations. Work at that end of the scale demands a different standard of attention: the detailing, the compliance trail and the coordination with specialist engineers all have to be right, because the consequences of getting them wrong are exactly what the importance level is measuring.
To be clear about roles: as an LBP Design Class designer I lead the architectural and building-code documentation and coordinate the project team. Where authority to approve falls outside my licensing, I bring in specialised personnel. The structural design for higher importance level buildings is always engineered and signed off by a CPEng structural engineer. That division of responsibility is normal, and on IL3 and IL4 work it is essential rather than optional.
If you are planning, buying or upgrading a building where importance level is in play, it helps to have someone who has documented that class of work before and knows how it moves through consent. You can read more about my background and LBP credentials on the about page.