Retrofitting double glazing involves installing secondary glazing or replacing existing single-glazed units with double-glazed sealed units, and costs typically range from £500 to £3,000 depending on window size and type (Energy Saving Trust, 2026).
The best method depends on your current window frame condition and budget. If your frames are sound, retrofitting new double-glazed sealed units into existing frames is often the most cost-effective route. If frames are damaged or you want minimal disruption, secondary glazing, adding a second pane on the inside, is a viable alternative. Neither approach requires full frame replacement, which keeps costs lower.
Retrofit sealed units into existing frames
This method replaces the single glass pane with a double-glazed sealed unit, keeping your existing frames. A professional glazier measures the frame aperture and orders a custom sealed unit, typically 4mm glass, 12mm argon-filled gap, and 4mm glass (BRE, 2026). The old glass is removed, the frame rebate is cleaned, and the new unit is fitted with new gaskets or sealant. Costs range from £150 to £500 per window, with a typical U-value of 1.6–2.0 W/m²K (Energy Saving Trust, 2026). This works best on timber or uPVC frames in good condition.
Install secondary glazing as a retrofit
Secondary glazing adds a separate pane on the inside of your existing single-glazed window, creating an insulating air gap. Systems range from magnetic frames that attach to the window reveal to sliding or hinged panels. Typical cost is £200–£600 per window, with a U-value improvement to around 2.5 W/m²K (UK Government Approved Document L, 2026). Secondary glazing is ideal for listed buildings or conservation areas where external changes are restricted. It also reduces noise by up to 40 decibels (BRE, 2026).
Check building regulations and grants
In England and Wales, replacement glazing must meet Building Regulations Part L, requiring a centre-pane U-value of 1.2 W/m²K or better (UK Government Approved Document L, 2026). Retrofit sealed units must be installed by a FENSA-registered or Competent Person Scheme member to avoid separate building control approval. For secondary glazing, no formal approval is needed unless the property is listed. The Great British Insulation Scheme closed on 31 March 2026. It funded insulation measures only and never covered windows or glazing. Always check eligibility before ordering.
A worked example
Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6). The existing timber frames are sound, so a certified glazier measures each aperture and orders custom sealed units, typically 4mm glass, 12mm argon gap, and 4mm glass, achieving a U-value of 1.8 W/m²K. The Energy Saving Trust estimates that upgrading from single glazing to this standard saves around £195 per year on heating bills in a gas-heated home. Over 25 years, that adds up to £4,875 in savings, easily covering the upfront cost. Homeowners on means-tested benefits may qualify for the ECO4 scheme, which can fully fund this work. For those not eligible for ECO4, the payback period is roughly 18 years, but the improved comfort, reduced condensation, and higher EPC rating add value beyond the cash savings.
| Item | Figure |
|---|---|
| Upfront cost after grants | £3,500 |
| Yearly savings | £195 |
| Payback period | 18 years |
| 25-year lifetime savings | £4,875 |
What homeowners often get wrong
The most common mistake is assuming you must replace the entire window frame to get double glazing, which can triple the cost unnecessarily. Here are three frequent errors homeowners make when retrofitting double glazing.
- Ignoring frame condition Many homeowners retrofit sealed units into rotten timber frames, and within two years the new glazing fails because the frame cannot support the extra weight or seal properly. A professional survey from a MCS-certified installer costs around £100 and identifies whether the frame is sound enough for a retrofit.
- Choosing the wrong U-value Some installers fit cheap sealed units with a U-value above 2.0 W/m²K, which saves only £80 per year instead of the £195 possible with a 1.6 unit. Specifying a U-value of 1.6 or lower in your contract ensures you get full energy savings and compliance with Building Regulations Part L.
- Skipping ventilation checks Retrofitting double glazing without adding trickle vents can cause severe condensation and mould, because the home becomes too airtight. Building Regulations require background ventilation in any room with replacement glazing, and missing this can void your warranty and create health risks.
Quick reference
- Retrofit double-glazed sealed units cost £150 to £500 per window, while secondary glazing costs £50 to £200 per window.
- A typical U-value for retrofit sealed units is 1.6 to 2.0 W/m²K, compared to 5.0 W/m²K for single glazing.
- You must have sound existing frames, timber or uPVC, for a retrofit sealed unit to be viable and warrantied.
- The Energy Saving Trust estimates annual savings of £195 for a gas-heated semi-detached house upgrading from single to double glazing.
- Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6).
Frequently Asked Questions
Yes, you can retrofit double-glazed sealed units into sound existing timber or uPVC frames. The Energy Saving Trust confirms this costs £150–£500 per window.
Secondary glazing is the cheapest retrofit option, starting from around £500 for a typical home. According to the Energy Saving Trust, it adds a pane inside your existing window.
Retrofitting double glazing typically costs £500 to £3,000 in the UK, depending on window size and type. The Energy Saving Trust provides these cost estimates for 2026.
