Earthquake strengthening is the process of upgrading an existing building’s structure so it can better resist seismic forces. In New Zealand, this is not theoretical. The Canterbury earthquakes, the Kaikoura earthquake, and ongoing seismic activity across the country have demonstrated what happens when buildings are not strong enough. For building owners in Dunedin and Otago, the stakes became more immediate in September 2025 when the Government announced that the region’s seismic hazard zone will shift from low to medium under proposed legislation, which has practical implications for property owners across the region. We deliver earthquake strengthening for residential and commercial buildings across Dunedin, and this guide covers what the process involves, who it affects, and what building owners need to understand before making decisions.
If you own a building in Dunedin that was built before 1976 (the year seismic design standards were first meaningfully applied in New Zealand), or if you have received an earthquake-prone building notice from Dunedin City Council, this article is written for you.
Why Does Earthquake Strengthening Matter in New Zealand?
New Zealand sits on the boundary of the Pacific and Australian tectonic plates. The Alpine Fault, the Hikurangi subduction zone, and hundreds of smaller fault systems generate ongoing seismic activity that affects every part of the country, including regions that were historically considered low-risk.
The Building (Earthquake-prone Buildings) Amendment Act 2016 created a national framework for identifying and managing earthquake-prone buildings. Under this legislation, territorial authorities (councils) are required to identify buildings that are below 34 percent of New Building Standard (NBS) and issue earthquake-prone building (EPB) notices. Building owners then have specified timeframes to either strengthen or demolish those buildings.
The consequences of inaction are real:
- Buildings below 34% NBS carry a legal obligation to remediate within set deadlines
- EPB notices are recorded on Land Information Memoranda (LIMs), affecting property values and insurance availability
- Banks are increasingly reluctant to lend against earthquake-prone properties
- Tenants, both residential and commercial, have legitimate safety concerns about occupying buildings that have not been assessed or strengthened
- In the event of a significant earthquake, under-strength buildings pose the greatest risk to life
This is not a Wellington or Christchurch problem. It is a New Zealand-wide obligation, and the proposed Otago zone reclassification has brought this into sharp focus for Dunedin building owners.
What Changed for Dunedin in September 2025?
In September 2025, the Cabinet Economic Policy Committee approved proposed changes to New Zealand’s earthquake-prone building (EPB) system. Based on the 2022 National Seismic Hazard Model, the Government announced that Coastal Otago, including Dunedin, will shift from a low to a medium seismic zone under proposed legislation (the Building (Earthquake-prone Buildings) Amendment Bill, introduced December 2025). The Bill is expected to be enacted by late 2026. Here is what this means for building owners:
Existing deadlines are preserved. The Cabinet decision expressly provides that EPBs in Coastal Otago issued an EPB notice before the Amendment Act commences will retain their current remediation deadlines. Existing timeframes are not being compressed. However, new EPB identifications in Dunedin going forward will follow medium-zone timeframes (25 years for non-priority buildings, 12.5 years for priority buildings).
More buildings may be identified. Moving to a medium zone means around 150 existing Dunedin EPBs remain in scope, and additional buildings may be identified in future. The revised system focuses on unreinforced masonry (URM) buildings and high-risk concrete buildings of three or more storeys.
Insurance and lending implications. Insurers and banks may factor the zone change into their risk assessments for Dunedin properties over time. Building owners who can demonstrate their buildings have been assessed and strengthened will be in a stronger position commercially.
Planning ahead matters. Dunedin’s pool of qualified seismic strengthening builders and structural engineers is limited. The assessment, design, consent, and construction process for a commercial strengthening project can take two to four years. Starting early gives you more options.
Which Buildings Need Earthquake Strengthening?
Not every building requires strengthening. The legislation targets buildings that are below 34% NBS, which is the threshold below which a building is legally classified as earthquake-prone.
Building types most likely to require assessment and strengthening in Dunedin:
| Building Type | Why It Is At Risk | Typical Strengthening Approach |
|---|---|---|
| Unreinforced masonry (URM) | Brick and stone walls with no steel reinforcement. Common in Dunedin’s central city and warehouse district. Most vulnerable building type in NZ earthquakes. | Steel frames, post-tensioning, FRP wrapping, base isolation, partial demolition and rebuild of critical elements |
| Pre-1976 reinforced concrete | Designed before modern seismic codes. Often has insufficient ductility in columns and beam-column joints. | Carbon fibre wrapping, steel jacketing of columns, new shear walls, foundation strengthening |
| Pre-1976 timber-framed residential | Older timber homes may lack bracing, have inadequate foundations (unreinforced concrete perimeter or timber piles), and have heavy cladding (brick veneer, stucco) on light frames. | Bracing upgrades, foundation bolting, pile replacement, chimney removal or restraint |
| Mixed-use heritage buildings | Dunedin’s heritage commercial buildings often combine URM, timber floors, and ad-hoc modifications over decades. | Comprehensive structural assessment followed by targeted strengthening of weakest elements. Often combined with adaptive reuse projects. |
The starting point for any building owner is a seismic assessment. An Initial Seismic Assessment (ISA) provides a preliminary estimate, and a Detailed Seismic Assessment (DSA) provides the legally relevant rating. This determines the building’s current %NBS rating and identifies the critical structural weaknesses that need to be addressed.
How Does the Earthquake Strengthening Process Work?
Seismic strengthening is not a single activity. It is a structured process that moves through assessment, design, consent, and construction. Understanding the stages helps building owners plan realistic timelines and budgets.
Stage 1: Initial Seismic Assessment (ISA)
A desktop assessment conducted by a structural engineer to estimate the building’s likely %NBS rating. This uses available information (original drawings, visible construction type, building age) to produce a preliminary score. Cost: typically $3,000 to $8,000. Duration: 2 to 4 weeks.
If the ISA indicates the building is likely below 34% NBS, a Detailed Seismic Assessment is recommended.
Stage 2: Detailed Seismic Assessment (DSA)
A thorough, invasive assessment that involves opening up walls and floors, testing material strengths, and modelling the building’s behaviour under seismic loading. The DSA produces an accurate %NBS rating and identifies specific structural deficiencies. Cost: $10,000 to $40,000+ depending on building size and complexity. Duration: 4 to 12 weeks.
The DSA is the key technical document. It provides the accurate %NBS rating that informs the council’s determination of whether your building is earthquake-prone, and it guides the strengthening design. The council, not the assessment alone, determines your legal obligations.
Stage 3: Strengthening design
A structural engineer designs the strengthening solution based on the DSA findings. The legal threshold for an EPB designation is 34% NBS. Many owners choose to strengthen to 67% NBS or higher, which provides a substantial margin above the EPB threshold and is the level most engineers recommend for long-term resilience. The design must balance structural performance, buildability, cost, and (for heritage buildings) preservation of character.
This stage involves close collaboration between the structural engineer and the builder. A strengthening design that looks elegant on paper but is impractical to construct in the existing building creates cost overruns and programme delays. At Gray Brothers, we work alongside structural engineers during the design phase to ensure the proposed solution is buildable within the real constraints of the existing structure.
Stage 4: Building consent
Earthquake strengthening work requires building consent from the territorial authority. In Dunedin, this means Dunedin City Council. The consent application includes the strengthening design, structural calculations, and a construction methodology. For heritage-listed buildings, resource consent and Heritage New Zealand approval may also be required.
Consent processing for seismic strengthening projects at DCC typically takes 6 to 16 weeks. Complex projects involving heritage considerations or peer review of structural design take longer.
Stage 5: Construction
The physical strengthening work. This varies enormously depending on the building type and the strengthening approach, but typically includes some combination of: installing new structural steel frames, adding reinforced concrete shear walls, strengthening foundations, securing parapets and facades, installing ties and diaphragm connections, and upgrading beam-column joints.
Construction duration ranges from 4 weeks for straightforward residential bracing upgrades to 12 months or more for major commercial strengthening projects.
Stage 6: Code Compliance Certificate
After construction, the council inspects the completed work and issues a CCC confirming the building now meets the consented strengthening design. The building’s %NBS rating is updated in council records.
Learn more about how we manage construction projects through our structured process.
What Are the Main Strengthening Methods Used in New Zealand?
The strengthening approach depends on the building type, the structural deficiencies identified in the DSA, and the target %NBS. Here are the methods most commonly used in Dunedin:
Structural steel frames. New steel moment frames or braced frames installed within or alongside the existing structure. These provide lateral resistance that the original building lacks. Common in URM buildings and pre-1976 concrete structures.
Reinforced concrete shear walls. New concrete walls poured within the existing structure to resist lateral forces. Often used in combination with steel frames for larger buildings.
FRP (Fibre Reinforced Polymer) wrapping. Carbon fibre or glass fibre sheets bonded to existing concrete or masonry elements to increase their strength and ductility. A less invasive option for concrete columns and walls that need additional capacity without major structural modification.
Post-tensioning. Steel tendons installed through the building’s structure and tensioned to compress masonry or concrete elements, improving their resistance to lateral forces. Commonly used on URM buildings where the masonry is in good condition but lacks tensile strength.
Foundation strengthening. Underpinning, new concrete foundations, or micro-pile installation to improve the connection between the building and the ground. Critical for buildings where the existing foundations cannot transfer seismic loads effectively.
Diaphragm connections. Installing steel ties and connectors between floors, walls, and roof structures to ensure the building acts as a connected system under seismic loading rather than separating at the connections.
Chimney removal or restraint. Unreinforced brick chimneys are one of the highest-risk elements in older residential buildings. Removing or restraining chimneys is often the single most effective life-safety improvement for pre-1976 Dunedin homes.
Residential vs Commercial Earthquake Strengthening
The legislation and the practical approach differ between residential and commercial buildings.
| Factor | Residential | Commercial |
|---|---|---|
| Typical building type | Timber-framed, 1-2 storeys, pre-1976 | URM, concrete, 2-4 storeys, pre-1976 |
| Common deficiencies | Inadequate bracing, poor foundation connections, heavy chimneys, brick veneer cladding | No lateral resistance, weak beam-column joints, unsecured parapets, inadequate diaphragms |
| EPB regime scope | Most ordinary residential timber homes fall outside the EPB regime. Voluntary strengthening is still worthwhile for safety. | URM and pre-1976 concrete buildings are the primary targets of the EPB system and frequently require remediation. |
| Typical strengthening cost | $15,000 to $80,000 | $100,000 to $1,000,000+ |
| Consent complexity | Standard building consent | Building consent + potential resource consent + Heritage NZ involvement depending on listing type |
| Occupancy during work | Often possible for minor work; may need to vacate for major strengthening | Often requires staged construction around tenants |
For commercial building owners, earthquake strengthening is often combined with a broader building upgrade or adaptive reuse project. Strengthening a heritage commercial building while simultaneously converting it to modern office, hospitality, or residential use can make the strengthening investment commercially viable. We have experience delivering these combined projects, including work on Dunedin’s commercial building stock. For detailed pricing on commercial projects, see the cost of a light commercial building project in Dunedin.
What Are the Regulatory Requirements?
The legal framework for earthquake-prone buildings in New Zealand involves several layers:
The Building (Earthquake-prone Buildings) Amendment Act 2016. This is the primary legislation. It requires territorial authorities to identify earthquake-prone buildings and issue EPB notices. Building owners must then remediate within specified timeframes.
Timeframes. Under the proposed reforms, buildings in medium seismic risk areas (which will include Dunedin once the Amendment Bill is enacted) have 25 years from the date of the EPB notice to complete strengthening or demolition. For priority buildings (URM buildings with facades that could fall onto busy routes, or buildings that could block emergency access), the timeframe is 12.5 years. Existing Dunedin EPBs issued a notice before the new law commences will retain their current deadlines. These timeframes sound long, but the assessment, design, consent, and construction process for a commercial strengthening project can take two to four years.
The %NBS threshold (current law). Under the current Building (Earthquake-prone Buildings) Amendment Act 2016, 34% NBS is the earthquake-prone threshold. Buildings assessed below this are subject to EPB notices and remediation timeframes. The target for strengthening work is typically 67% NBS or higher, which removes the EPB classification and provides a reasonable margin of safety. The proposed reforms will change how buildings are identified and categorised, but the 34% threshold remains the baseline reference point.
Heritage considerations. Heritage-listed buildings face the same strengthening obligations but may have additional requirements depending on their listing type and any district plan protections. Resource consent may be required for exterior modifications, and an archaeological authority may be needed for pre-1900 sites. Where Heritage New Zealand is involved, the choice of strengthening methods and processing time may be affected. Check your building’s specific protections early.
For the most current information on earthquake-prone building requirements, refer to the Ministry of Business, Innovation and Employment (MBIE) guidance.
Should You Strengthen, Demolish, or Sell?
This is the decision framework every owner of an earthquake-prone building faces.
Strengthen when: the building is structurally viable, the strengthening cost is justified by the property’s value and income potential, and the building has a useful economic life of 25+ years post-strengthening. This is the right choice for well-located commercial buildings in Dunedin’s central city, heritage buildings worth preserving, and residential properties in desirable suburbs. Understanding the cost of building a custom home in Dunedin helps residential owners weigh strengthening against rebuilding.
Demolish when: the strengthening cost exceeds the building’s post-strengthened value, the building has other fundamental deficiencies (severe weather-tightness failure, contamination, major structural deterioration beyond seismic issues), or the site value significantly exceeds the improved building value.
Sell when: you do not want to manage a strengthening project yourself, and there is a buyer willing to take on the obligation. Be aware that EPB notices on the LIM significantly reduce market value and limit the buyer pool.
An honest builder will tell you which option makes sense for your specific building. If strengthening does not make financial sense, we will say so rather than quoting work that delivers a poor return on your investment.
How to Choose a Builder for Earthquake Strengthening
Seismic strengthening is specialised construction work that not every builder is equipped to deliver. When choosing a builder for this work:
Verify specific seismic strengthening experience. Ask for completed strengthening projects, not just general construction portfolios. The structural challenges of working within existing buildings under seismic loads are fundamentally different from new-build construction.
Check LBP status. For residential buildings, seismic strengthening that affects the primary structure is restricted building work and must be carried out or supervised by a Licensed Building Practitioner. Commercial projects have different requirements but using an LBP-licensed builder is best practice regardless. Check the LBP register.
Ask about their relationship with structural engineers. Effective strengthening requires close collaboration between the builder and the structural engineer throughout design and construction. A builder who works regularly with seismic engineers delivers better outcomes than one who treats the engineer’s drawings as a fixed instruction set.
Understand their approach to heritage. If your building is heritage-listed, the builder needs experience working within Heritage New Zealand requirements. This influences material choices, construction methods, and the overall approach to preserving character while achieving structural targets.
Ask about project management. Strengthening projects in occupied buildings require careful staging, communication with tenants, and programme management. How will the builder manage disruption?
Learn more about who we are and how we approach complex construction.
How Gray Brothers Builders Approaches Earthquake Strengthening
We have been building and strengthening structures in Dunedin for over 50 years. Our team works across both residential and commercial seismic strengthening, from chimney removal and bracing upgrades on family homes through to major structural interventions on commercial buildings.
The Hanover Street Commercial Apartments project demonstrates our approach to working within existing commercial structures: understanding the building’s original construction, collaborating closely with engineers on practical strengthening solutions, and managing construction within the realities of a central-city site.
Every strengthening project starts with an honest assessment. We look at the building’s condition, the likely strengthening scope, the realistic cost, and whether the investment makes sense for the building owner. Not every building should be strengthened. Some should be demolished. Some should be sold. Our job is to give you the information to make the right decision, not to sell you a project.
Our structured process applies to seismic work the same way it applies to every project: thorough pre-construction assessment, transparent pricing, clear communication, and a 287-point quality checklist that ensures nothing is missed.
Frequently Asked Questions
How do I know if my building is earthquake-prone?
Check your property’s LIM from Dunedin City Council. If an EPB notice has been issued, it will appear on the LIM. You can also commission an Initial Seismic Assessment from a structural engineer to get a preliminary %NBS rating before council identifies your building.
What does earthquake strengthening cost for a residential home in Dunedin?
Residential strengthening typically costs $15,000 to $80,000 depending on the scope. Chimney removal or restraint is at the lower end. Full bracing upgrades with foundation work sit at the higher end. Read our dedicated cost of earthquake strengthening in New Zealand guide for detailed pricing.
How long does earthquake strengthening take?
Residential bracing and chimney work: 2 to 6 weeks on site. Commercial strengthening: 3 to 12 months depending on building size and complexity. Add 3 to 6 months for assessment, design, and consent before construction begins.
Can I live in my house during strengthening work?
For minor work (chimney removal, external bracing), yes. For major foundation or structural work that affects the building’s stability during construction, temporary relocation is usually necessary.
Does earthquake strengthening increase my property value?
Yes, in most cases. Removing an EPB notice from a property’s LIM improves its marketability, insurability, and ability to secure bank lending. The value increase is typically greater for commercial properties than residential.
Not Sure Where Your Building Stands?
The most useful first step is understanding where your building stands. Whether you have received an EPB notice, are concerned about the Otago zone reclassification, or simply want to know what your building’s seismic status is, a conversation with a builder experienced in strengthening work will give you clarity.
Contact Gray Brothers Builders to discuss your building. We will assess the situation honestly, explain your options, and help you decide whether strengthening, demolition, or doing nothing is the right call for your property. No obligation, no pressure. Just straight answers from a team that has been working with Dunedin’s building stock for more than 50 years.