How do I choose a double glazing company?
Short answer
Retrofit double glazing is a joinery job with long warranties attached, so the company matters as much as the glass. Look for experience with your window type, glazing that meets the NZS 4223 safety standard where it applies, and written warranties for the sealed units and the installation separately. Like-for-like replacement rarely needs consent; changing the openings does.
Source: MBIE Building Performance (exempt building work). Updated June 2026.
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- Replacing glazing in existing openings is usually exempt building work; resizing or moving windows needs consent
- NZS 4223 sets where safety glass is required, including doors, low windows and bathrooms
- Sealed unit warranties commonly run 5 to 10 years and are claimed through the installer
- Retrofit options range from new sealed units in your existing frames to full joinery replacement
Know which job you are buying
Double glazing covers three different jobs: fitting new sealed units into your existing timber or aluminium frames, replacing the whole window joinery, and secondary glazing added inside the existing window. They differ enormously in price and disruption, and most companies specialise in one of them. A company that mainly replaces joinery will quote you joinery replacement, so decide which job you want before you collect quotes, or ask each company to price the same one.
Secondary glazing is the budget path of the three and can suit heritage windows that cannot be altered.
Consent and standards
Swapping glazing or windows on a like-for-like basis in existing openings is usually exempt building work. The moment a quote involves enlarging, moving or adding openings, building consent enters the picture, and your council is the right place to confirm what applies to your house.
Wherever new glass goes in, NZS 4223 decides where safety glass is mandatory. A good company works this out room by room as part of the quote rather than treating it as an extra.
If the glazing is part of a larger renovation, the H1 insulation requirements in the Building Code may shape what the new windows have to achieve, which is another question your council can answer before you commit.
Comparing quotes properly
Glass specifications move the price and the performance, so get them in writing: low-E coating or not, argon fill or air, and the spacer type between panes. Ask who measures, who installs, and whether the installers are the company's own people.
Then ask for both warranties separately. The sealed units carry a manufacturer warranty, typically 5 to 10 years against seal failure, and the installation carries a workmanship warranty from the company. Condensation forming between the panes later is the classic seal failure claim, so know now who you would call.
The warranty is only as good as the company
A 10 year warranty only helps if someone is still answering the phone in year eight, which makes the company's history part of the product. Membership of the Window and Glass Association NZ is a reasonable signal of an established operator, and a few minutes looking into how long the company has traded under its current name is time well spent before you pay a deposit.
Knowing the rules is half the job. The other half is knowing who you're hiring. Check any NZ builder against the public record: company status, licensing and insolvency notices, from the official NZ sources.
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Related questions
Sources: MBIE Building Performance (exempt building work); Window and Glass Association NZ; Consumer NZ. General information for NZ homeowners, not legal advice. Building rules change and vary by council, so confirm critical details on the official source before acting. Last updated 2026-06.