Retrofitting double glazing into a heritage-listed Melbourne home raises questions that don't apply to standard residential jobs: council permits, heritage overlay conditions, character window preservation, and glass specifications that match the original aesthetic.
This guide covers what is typically permitted, what requires a permit, and how to keep your original timber frames while achieving modern thermal performance.
Understanding Heritage Overlays
Melbourne's heritage framework operates at two levels:
State-listed heritage (Heritage Victoria register): Highest level of protection. Any works affecting external fabric typically require a permit from Heritage Victoria.
Local heritage overlays (council HO or NDO): More common. Council-administered. Requirements vary significantly between councils and even between individual properties within the same overlay.
The critical point: IGU retrofit into existing frames generally does not alter the external appearance of the building, since the glass change is not visible from outside. Many councils' heritage overlays explicitly permit internal changes that do not affect external character.
What Usually Requires a Permit
- Full window replacement (changing the frame, sash, or hardware profile)
- Adding double-hung mechanisms to original fixed lights
- Changing glass from clear to tinted (potentially visible from street)
- Structural works to window openings
What Usually Does Not Require a Permit
- Retrofit IGU into existing frame and sash (glass change only)
- Like-for-like glass replacement maintaining the same profile
- Resealing and repainting existing frames
Always verify with your council's planning department before proceeding. This guide provides general information only.
Retrofit Into Timber: How It Works
Traditional timber window frames were built with single glass in mind — the rebate (the channel the glass sits in) is typically shallower than modern aluminium. In practice, most Victorian and Edwardian-era timber frames accommodate a 20–24mm IGU with either:
Direct fit: The rebate depth is already sufficient. IGU seats in existing glazing tape and is secured with timber or aluminium beads.
Rebate extension: The existing rebate is deepened by a few millimetres using a router, allowing the IGU to seat correctly. A small, invisible modification.
Replacement sash: Where frame depth truly cannot accommodate the IGU without structural compromise, a new sash matching the original profile is fabricated and installed in the existing frame. Common on very narrow heritage window styles.
Aesthetics: What Changes, What Doesn't
From the street, the window looks identical. The glass may appear very slightly different in reflectivity depending on whether Low-E coating is used, but this is not typically visible from a normal viewing distance.
Inside, the window operates identically — same sash, same hardware, same opening mechanism.
Colonial bars and astragals: Where windows have decorative dividing bars, these can be retained in the original frame. For sashes that require replacement, we fabricate matching profiles.
Recommended Specification for Heritage Homes
For most heritage homes, hard-coat Low-E in a 24mm IGU is the right balance: meaningful thermal improvement (U-value down to ~1.9), authentic appearance, and no argon gas fill to complicate the narrower IGU configurations some heritage sashes require.
Acoustic performance matters in inner-Melbourne heritage suburbs (Fitzroy, Carlton, Richmond, Clifton Hill). Adding acoustic laminated glass as the inner pane is compatible with heritage retrofit and adds Rw 40–44 performance.
Getting a Heritage Assessment
The Heritage Double Glazing page covers the full process. For a pricing indication before committing to anything, the Instant Estimate tool works for heritage retrofits the same as any other job.
Related: Is retrofit double glazing worth it?
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