What's the definition of deep retrofit?
Author Ingo Ratsdorf, Senior Architect | Design Team Leader | BE Arch. Reg Arch (NZ,DE) Building Biologist (NZ,DE). 2026
What's the definition of deep retrofit?
A deep retrofit refers to a comprehensive renovation of an existing building aimed at dramatically improving its energy performance, typically targeting reductions in energy use of 50% or more compared to the building's pre-retrofit baseline (some definitions set the bar even higher, around 70-90%, especially for net-zero-ready outcomes).
Unlike a standard retrofit, which might involve swapping out a few inefficient components like old light fixtures or a single HVAC unit, a deep retrofit takes a whole-building approach. It usually addresses multiple systems at once, such as the building envelope (insulation, windows, air sealing), heating and cooling systems, ventilation, lighting, and sometimes on-site renewable energy generation like solar panels. The idea is to treat the building as an integrated system rather than upgrading pieces in isolation, since improvements to one area (like better insulation) can change the requirements for another (like a smaller, more efficient HVAC system).
Deep retrofits are often pursued for older or aging building stock, particularly in contexts like multifamily housing, commercial offices, or public buildings, where the existing systems are inefficient and a full overhaul makes more financial and environmental sense than incremental fixes over time. They tend to be more capital-intensive upfront than shallow or "light touch" retrofits, but are designed to deliver much larger long-term savings on energy costs and emissions, and can also improve occupant comfort, indoor air quality, and building durability.
The term is sometimes paired with related concepts like "deep energy retrofit" (used somewhat interchangeably) or tied to specific certification programs and government incentive schemes that set their own thresholds for what counts as "deep."
Building Stock
There isn't a single official "total buildings" count that gets published the way population figures do, since it depends on what counts as a building, but a few useful numbers exist.
For homes specifically, Stats NZ's most recent dwelling estimate puts the figure at 2,141,200 private dwellings as of 31 December 2025. This is the figure most commonly cited when people ask about "how many buildings" in a housing context, since it's updated quarterly and is methodologically solid (based on census counts plus building consent data).
For context on a narrower slice, an earlier industry estimate put the number of commercial (office, retail, and mixed-use) buildings at more than 41,000, with a total floor area of nearly 40 million square metres.
How much of the current New Zealand building stock is estimated to be still standing in 2050?
There isn't a single official New Zealand-specific percentage for this, but the figures that exist point clearly in one direction: the overwhelming majority of buildings standing in 2050 already exist today.
The most directly relevant New Zealand analysis comes from MBIE's Building and Construction Sector Trends reporting, which tackled this question to estimate the scale of retrofit needed to hit the 2050 target. Rather than producing its own bespoke percentage, it reasoned from building lifespan data: residential buildings can have a 70 to 100 year lifespan, and in New Zealand, buildings from the 1920s are still in use, which suggests a 100-year lifespan is realistic for a meaningful share of the stock. Based on this, MBIE concluded it's plausible that the existing New Zealand building stock making up the 2050 stock could comprise buildings built from 1950 onwards, meaning almost all of what exists today (and a fair bit of what's already been demolished or will be replaced over the next 25 years) is the baseline for any retrofit conversation.
For an actual percentage, New Zealand reporting borrows from the International Energy Agency's global estimate, since no NZ-specific equivalent figure was located in the sources I could access: the IEA estimated that around two-thirds of the global building area that exists today would still exist in 2040. A commonly cited variant of this same having already been built, which is the figure you'll see most often in international retrofit and embodied-carbon discussions.
So in practice, New Zealand's own building and construction sector reporting treats the global figure (roughly two-thirds to 80%, depending on the source and timeframe) as a reasonable proxy for the local situation, rather than citing a distinct New Zealand percentage. The practical implication MBIE draws from this is that retrofit, not just new-build standards, has to do a lot of the heavy lifting to meet the building sector's net-zero pathway, since: a large share of the housing stock still lacks basics like double glazing or full insulation, with one data point being that only about 16% of dwellings had double-glazing on all windows as of 2018, while wall insulation was absent in 53% of houses as of a 2015 estimate; and new construction itself keeps adding meaningfully to the total stock, projected to be on the order of 8 million square metres a year, which means the "2050 stock" isn't static, it's a moving target shaped by both retention of old buildings and the rate of new ones.
Financial benefits of commercial retrofitting in NZ
Retrofitting delivers direct financial returns for commercial owners and occupiers. Upgraded buildings can cut energy costs by up to 50%, reducing long-term operating expenditure and increasing net returns[i]. Retrofitted assets also command up to a 6.7% rental premium and 9.8% higher sale value[ii] compared with unretrofitted stock, while facing vacancy rates as low as 1.5%, versus 6.9% for older buildings[iii], reflecting tenant demand for lower-cost, efficient space. Owners can further access discounted finance rates from banks for qualifying retrofit projects, lowering capital costs and improving investment returns.
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For clients retrofitting existing buildings, this experience spans multiple disciplines: Structural Engineering services including Initial and Detailed Seismic Assessments, and Architecture & Design covering adaptive reuse projects from feasibility through to building consent.
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References:
[1] Retrieved from https://www.stats.govt.nz/information-releases/dwelling-and-household-estimates-december-2025-quarter/ on 16 June 2026
[2] Retrieved from https://www.researchgate.net/figure/Number-and-area-of-New-Zealand-commercial-buildings_tbl3_328368581 on 16 June 2026
[3] Retrieved from https://www.mbie.govt.nz/building-and-energy/building/building-system-insights-programme/sector-trends-reporting/biannual-snapshots/november-2022 on 16 June 2026
[i] Formance, "Green Financing for Property Developers in New Zealand", retrieved 30/07/2026 from https://www.formance.co.nz/learn/green-financing-for-property-developers-in-new-zealand/
[ii] NZGBC, “Sustainable offices deliver better returns, new report finds“, retrieved 30/07/2026 from https://nzgbc.org.nz/news-and-media/sustainable-offices-deliver-better-returns-new-report-finds
[iii] NZ Herald, “Green buildings make money for landlords and tenants according to landmark new report”, retrieved 30/07/2026 from https://www.nzherald.co.nz/business/companies/banking-finance/green-buildings-make-money-for-landlords-and-tenants-according-to-landmark-new-report/Z6AXPSFLBRAAZF7P64VAXNGLAY/