The Building Act also includes provisions for existing buildings, with the intent that people can use buildings safely and the seismic performance of the building stock should improve over time. The Building Act does not require existing buildings to achieve compliance with the current Building Code. Structural upgrading of existing buildings is not required unless they are considered dangerous or earthquake-prone (Building Act sections 121 and 124).
However, new building work undertaken when the building is being altered needs to comply fully with the Building Act. The bottom line is that the building cannot have a lower standard of performance after alterations are completed than it had before. This requirement is set out in two places in the Act – Section 42A covers building work where a building consent is not required (under Schedule 1 of the Act). Section 112 applies where a consent is required. The structural aspects of the existing building may need to be upgraded if the building is undergoing a change of use (Building Act sections 114 and 115).
In terms of the legislative framework, earthquake strengthening is considered the same as standard building work, and all the same building control processes apply. Generally, strengthening work will use a specific engineering design for each case. It is the responsibility of the building owner or designer to prove that all the requirements of the Building Code are met. This may be done in a number of ways, including using Verification Methods, peer review, physical tests or special analysis, for example.
Change of use
Every building or part of a building is deemed to have been built for an intended use – see Schedule 2 of the Building (Specified Systems, Change the Use, and Earthquake-prone Buildings) Regulations 2005. The different uses relate to fire purpose groups. The Building Act therefore has specific requirements when the intended use of a building is to change.
When the intended use of a building changes, its structural performance must be upgraded if the Building Code performance requirements for the new intended use are more onerous than before the change. The upgrade must be done to a standard that is “as nearly as is reasonably practicable” to that of a new building that would be used for the same purpose, including earthquake resistance. The question of what is reasonably practicable is a trade-off between the advantages and disadvantages of upgrading the building to comply with any particular provision of the Building Code.
A building owner intending to change the use of an existing building must give written notice to the building consent authority (BCA). In order that building upgrade requirements are proportionate and not excessively costly for owners, BCAs are required to balance issues of safety and community interests against the cost of improvements.
Building standards have developed considerably over the years, and many existing buildings fall short of the standards now required for new buildings. The objective of the Building Act regarding earthquake-prone buildings is to identify the most vulnerable buildings and reduce the level of risk to people.
To do this, the Building Act establishes the concept of earthquake-prone buildings and provisions to improve these buildings over time. The Building Act (in section 133AB) defines an earthquake-prone building as follows:
(1) A building or a part of a building is earthquake prone if, having regard to the condition of the building or part and to the ground on which the building is built, and because of the construction of the building or part:
a. the building or part will have its ultimate capacity exceeded in a moderate earthquake; and
b. if the building or part were to collapse, the collapse would be likely to cause:
i. injury or death to persons in or near the building or on any other property; or
ii. damage to any other property.
(2) Whether a building or a part of a building is earthquake prone is determined by the territorial authority in whose district the building is situated.
The Building (Specified Systems, Change the Use, and Earthquake-prone Buildings) Regulations define a moderate earthquake as one that would generate shaking at the site of the building that is of the same duration but is one-third as strong as the earthquake shaking (as determined by normal measures of acceleration, velocity and displacement) that would be used to design a new building at that site if it were designed on 1 July 2017.
In practice, an earthquake-prone building is often referred to as one that meets 33% or less of the standard that a new building would be required to meet. This is sometimes called the one-third rule.
The definition of an earthquake-prone building takes into account a range of factors, including different levels of seismic risk around New Zealand. Therefore, a building in Wellington (which has a higher seismic risk) that meets 33% of the new building standard will be stronger in absolute terms than a building in Auckland (which has a lower seismic risk) that also meets 33% of the new building standard.
33% of the new building standard corresponds to approximately 20 times the risk of the building reaching a similar condition to what a new building at 100% of the new building standard would reach in a full design earthquake. Given this risk, many believe that strengthening to 66% (two-thirds) of the new building standard is reasonable and economically achievable for most buildings, although it exceeds the minimum statutory requirement.
A percentage of the new building standard is also sometimes used to describe the earthquake resilience of buildings in general, with higher ratings considered better. This is usually denoted as %NBS.
The Building Act includes a system for managing earthquake-prone buildings (set out in sections 133AA to 133AY of the Act). This aims to ensure there is a consistent approach to dealing with earthquake-prone buildings and helps provide information about the number and location of them around the country. The regime includes:
- universal timeframes for the identification and remediation of earthquake-prone buildings
- a standard methodology for assessing earthquake-prone buildings
- a national register of earthquake-prone buildings.
The provisions do not apply to individual family homes, but do apply to residential buildings that are at least 2 storeys and house three or more household units or are used as a hostel or boarding house. There are special provisions for certain heritage buildings.
Territorial authorities are obliged to identify potentially earthquake-prone buildings using the EPB methodology. In medium or high seismic risk areas, there are priority buildings that territorial authorities should identify first
When a territorial authority identifies a potentially earthquake-prone building, it must notify the building owner. The building owner must provide an engineering assessment of the building (in accordance with the EPB methodology) within 12 months. The engineering assessment must include an assessment of the %NBS of the building.
The territorial authority must then determine whether or not a building is earthquake-prone using the engineering assessments, property files and any other information provided by the building owner, in accordance with the tests set out in the EPB methodology. If the building is determined to be earthquake-prone, the territorial authority:
An owner of an earthquake-prone building must take action to either strengthen the building so it is no longer earthquake-prone or demolish it to remove the risk. The deadline for action is specified in the EPB notice and depends on whether the building is a priority building and the seismic risk area the building is located in.
The management system includes use of notices to inform the public of a building’s earthquake-prone status and the concepts of priority buildings and substantial alterations.
An EPB notice is intended to inform building users and the public about the potential seismic performance of an earthquake-prone building. The notice must be prominently displayed on or adjacent to the building where it can be easily seen.
EPB notices are issued by a territorial authority when it determines that a building or part of a building is earthquake-prone. The notice identifies the building or part of the building that is earthquake-prone and, where possible, provides the building’s current earthquake rating and the timeframe for remediation work to be completed.
There are three types of EPB notice, each with a distinctive border indicating the building’s earthquake rating.
The different borders indicate three different earthquake ratings: 0% to less than 20%NBS or no engineering assessment and no rating determined (left); 20% to less than 34%NBS (middle); and a section 124 notice, given before 1 July 2017 where no rating was known (right). (Image sourced from MBIE's Earthquake-prone buildings: notices)
A fourth type of EPB notice indicates that a territorial authority has granted an exemption from undertaking seismic work on an earthquake-prone building.
An EPB exemption notice. (Image sourced from MBIE's Earthquake-prone buildings: notices)
The exemption recognises that, while a building may be earthquake-prone, the risk to life and property is low.
MBIE’s Managing earthquake-prone buildings resource provides a full description of EPB notices and how to use them as well as a range of notice templates.
Two publications from 2022 also provide useful guidance: Seismic risk guidance for buildings (PDF) and Managing earthquake-prone council buildings – a decision framework.
Priority buildings are certain types of buildings in high and medium seismic risk areas that are considered to present a higher risk because of their construction, type, use or location. They need to be identified and remediated within half the time allowed for other buildings in the same seismic risk areas.
Certain hospital, emergency and education buildings are prioritised in the Building Act because of their function. Territorial authorities may also identify priority buildings on thoroughfares with high pedestrian and vehicle traffic.
MBIE’s updated Priority Buildings: A guide to the earthquake-prone building provisions of the Building Act provides full guidance for priority buildings.
If a building owner plans to carry out substantial alterations to an earthquake-prone building, they must now also complete seismic remediation work at the same time. If the remediation work is not done, the BCA cannot grant consent for the substantial alterations.
BCAs use a set of criteria to determine whether building work amounts to substantial alterations when they receive a building consent application.
Generally, this means substantial alterations are any work to an earthquake-prone building (or part of an earthquake-prone building) that:
- requires a building consent
- together with other consented work to the building over the past 2 years, has an estimated value of at least 25% of the building’s value.
MBIE’s Earthquake-prone buildings: substantial alterations information sheet explains the criteria BCAs must apply to decide whether building work amounts to substantial alterations.