Is double glazing required in NZ rentals?

No. Nothing in the Healthy Homes standards requires double glazing in an existing rental. The standards touch windows in two other ways: habitable rooms need windows or doors that open to the outside (ventilation), and unreasonable gaps around windows need sealing (draught stopping). The glass itself can be single-pane and the property can still comply on every standard.

Where the confusion comes from

New construction is different: the Building Code's insulation requirements have made double glazing effectively universal in new builds for years. People reasonably assume rental rules follow, but the Healthy Homes standards were written to be achievable across the existing housing stock, and reglazing a 1920s villa was never on that list. Window insulation does affect the heating calculation though: single glazing increases the heater capacity a living room needs, so the house pays for its glass one way or another.

When it is still worth doing

• Cold-zone properties where winter condensation streams down the glass every morning.
• Rooms where the heating calculation comes back demanding a notably large heater.
• Any planned joinery replacement, the marginal cost of better glass at that point is small.

If you do upgrade, file the invoice with the property's records: it changes the heating maths in your favour and it is exactly the kind of evidence a future compliance statement, insurance claim or sale benefits from.