Retrofit vs replacement windows in New Zealand
Retrofit costs $450–800 per m² of glass, replacement $800–1,600 (CheckMyBuilder). What each one actually changes, what stays, and which houses suit which.
By the NZ Double Glazing team
Key takeaways
- Retrofit keeps your frames and puts a sealed double-glazed unit into them: $450–800 per m² of glass (CheckMyBuilder, checked 15 Sep 2026).
- Full replacement takes the whole window out, frame included: $800–1,600 per m², so roughly double.
- Retrofit leaves the frame as it was, so a timber or plain aluminium frame is still the frame it was; only replacement can give you thermally broken joinery.
- Both are usually exempt from building consent in an existing house of up to two storeys (Building Act Schedule 1, Exemption 8), and both must still meet the Building Code.
- Character joinery is the strongest argument for retrofit; rotten or failing joinery is the strongest argument for replacement.
Retrofit double glazing keeps your existing window frames and fits a sealed double-glazed unit into them, at $450–800 per m² of glass. Full replacement removes the whole window, frame and all, at $800–1,600 per m² (CheckMyBuilder, checked 15 Sep 2026). Retrofit is cheaper and faster; replacement is the only route to a new, thermally broken frame.
What each one actually does
Retrofit works on the glass, not the house. The glazier takes the sash or the glass out, prepares the rebate to take a thicker sealed unit, and refits it. Your joinery stays where it is. Replacement takes the entire window assembly out — frame, sashes, sill flashing — and puts a new, factory-glazed window in its place.
| Retrofit | Full replacement | |
|---|---|---|
| Per m² of glass (CheckMyBuilder, retroGLAZE) | $450–800 | $800–1,600 |
| Per window / opening (Wellington Glass) | Aluminium $550–850, timber $800–1,200 | $1,500–2,500+ |
| Your frames | Kept | Gone |
| Thermally broken joinery possible | No | Yes (+25–40%, Superior Renovations) |
| Usually consent-exempt in an existing dwelling | Yes | Yes |
Why the published ranges vary so much
Three things move a published figure. First, whether it prices retrofit or replacement — that is the biggest gap, roughly double. Second, frame type: Wellington Glass separates aluminium retrofit ($550–850 a window) from timber ($800–1,200), because a timber sash takes more workshop time. Third, region and house: CheckMyBuilder puts a typical Auckland job at $18,000 against $16,000 nationally.
When retrofit is the right call
- Your joinery is sound. A glazier can only retrofit into a frame that will hold a heavier unit and still open, close and seal.
- You want to keep the look. On a villa or bungalow, the sash profile and the glazing bars are the character; retrofit keeps them.
- Budget is the deciding factor. At roughly half the per-m² rate, retrofit does more windows for the same money — and doing every window matters more than doing a few perfectly.
- You want it done quickly and with less disruption. Retrofit is usually a workshop job with short on-site visits, rather than a build.
When replacement is the right call
- The joinery has had it. Rot, failed seals, corroded aluminium or a window that no longer closes are not fixed by better glass.
- You want a thermally broken frame. Retrofit cannot give you one: the frame is the frame you already have. A thermally broken aluminium replacement adds 25–40% to the joinery cost (Superior Renovations, Sept 2026).
- You are changing the opening — a bigger window, a different style, a ranchslider where a window was.
- You are already in a renovation that has the linings off. The marginal cost of replacement is lowest when the wall is open anyway.
What neither of them fixes
Windows are one part of the envelope. BRANZ's Level guidance puts windows at 8–10% of the building envelope area, while EECA says up to 40% of a home's heating energy is lost through them, dropping to 25% or less with double glazing. Ceiling and underfloor insulation are cheaper per unit of heat saved, and Warmer Kiwi Homes co-funds those, not glazing (EECA).
How to decide in one afternoon
- Walk the house and write down every window: room, rough width and height in millimetres, and whether it opens.
- Look at the frames. Timber that is sound and painted, or aluminium that still slides and seals, both retrofit well.
- Price both routes on the same window list, so you are comparing the same house rather than two quotes with different scopes.
- Get a measure. The frame check is the part you cannot do from the kerb, and it is what turns a range into a fixed number.
Questions
Is retrofit double glazing as good as replacement?
For the glass, yes: both give you a sealed double-glazed unit. For the frame, no. Retrofit leaves your existing frame in place, so if that frame is a plain aluminium one it stays a plain aluminium one. Only replacement can put a thermally broken frame in.
How much cheaper is retrofit?
Roughly half per m² of glass: $450–800 against $800–1,600 (CheckMyBuilder, checked 15 Sep 2026). Per window, Wellington Glass publishes $550–850 for an aluminium retrofit against $1,500–2,500+ for a full replacement opening.
Can any frame be retrofitted?
No. The frame has to be sound and deep enough to take a thicker sealed unit, and the sash has to still work once it is heavier. That is exactly what the measure checks before you commit to anything.
Do I need building consent for either?
Usually not. Schedule 1 Exemption 8 of the Building Act covers window work in an existing dwelling of up to two storeys, provided the window being replaced has not failed its 15-year durability requirement. Exempt work still has to comply with the Building Code. See our consent guide.
Will retrofit stop my windows opening properly?
A sealed unit is heavier than a single pane, so hinges, stays and sash cords sometimes need attention. A good glazier flags that at the measure and prices it, rather than discovering it on install day.
Which should I do if I can only afford some of the house?
Do the rooms you live in and sleep in first, and price the rest so you know the number. Doing every window at retrofit rates usually beats doing a third of them at replacement rates, unless the joinery is failing.
Sources
Published 15 September 2026, last updated 15 September 2026 by the NZ Double Glazing team.