Timber - PVC - Aluminium - Windows, Doors & Conservatories in Hampshire
Double Glazing for Listed Buildings: The 2025 Planning Guide
Double Glazing for Listed Buildings: The 2025 Planning Guide
Category: Heritage & Conservation
Reading Time: 10 Minutes
📌 2025 Planning Update: Key Takeaways
- The Old Rule: “No double glazing in Grade II buildings” is no longer an absolute rule.
- The Game Changer: “Vacuum Glazing” (like LandVac/Fineo) is now thin enough (6.7mm) to fit historic frames.
- Planning Policy: Historic England’s latest guidance supports retrofitting if it doesn’t harm the “significance” of the building.
- The Alternative: Secondary glazing remains the best planning-free option for Grade I or strict areas.
For decades, owning a Listed Building in the UK meant resigning yourself to cold winters, condensation, and sky-high energy bills. The stance from Conservation Officers was usually a strict “no” to double glazing.
However, the tide is turning. With the urgent need to decarbonise UK homes and rapid advancements in glass technology, 2025 is seeing a shift in planning attitudes. It is no longer impossible to get Listed Building Consent for double glazing—if you use the right technology.
Table of Contents
1. The Historic Problem: “Thick” Glass
Why have Conservation Officers historically rejected double glazing? It wasn’t just about the material; it was about the sightlines.
Standard double glazing is around 24mm to 28mm thick. Original single-glazed Georgian or Victorian sash windows are designed for glass that is only 3mm or 4mm thick. Trying to squeeze a fat double-glazed unit into a delicate timber frame requires deepening the rebate and making the glazing bars (muntins) much thicker. This destroys the delicate, elegant proportions of the historic window, leading to the “clunky” look that planners hate.
2. The 2025 Solution: Vacuum Glazing
This is the technology that is changing planning decisions across Hampshire and the UK. Vacuum Glazing (such as LandVac or Fineo) works differently from standard double glazing.
[Image of vacuum glazing cross section diagram]Instead of filling the gap between the panes with gas (Argon/Krypton), all the air is sucked out to create a vacuum. Because a vacuum is the perfect insulator (heat cannot travel through nothing), the gap between the panes can be microscopic—just 0.3mm.
| Feature | Single Glazing (Historic) | Standard Double Glazing | Vacuum Glazing (LandVac) |
|---|---|---|---|
| Total Thickness | 4mm | 24mm – 28mm | 8.3mm |
| U-Value (Insulation) | 5.8 (Very Poor) | 1.2 (Good) | 0.4 – 0.7 (Excellent) |
| Visual Impact | Original | Thick Sightlines | Virtually Invisible |
3. The Alternative: Slimline Double Glazing
Before vacuum glass became widely available, “Slimline” double glazing was the best option for heritage homes. These units use a smaller gap (4mm-6mm) filled with heavy gases like Krypton or Xenon to achieve insulation.
While thinner than standard units (typically 12mm-14mm total), they are still thicker than single glazing. They remain a valid option for many conservation areas, especially if vacuum glazing is outside your budget, but they generally have a shorter warranty due to the reduced seal depth.
4. Navigating Listed Building Consent
Even with this new technology, you must apply for Listed Building Consent (LBC). Changing windows without it is a criminal offence. However, your case is now stronger than ever.
Historic England’s Stance: In their recent guidance on “Traditional Windows: Their Care, Repair and Upgrading,” Historic England acknowledges that upgrading glazing can be acceptable where it does not harm significance.
5. The Safe Bet: Secondary Glazing
If your property is Grade I listed, or you want to avoid the planning headache entirely, Secondary Glazing remains the ultimate solution.
Unlike replacing the window, secondary glazing is a reversible alteration. You add a slimline aluminium frame on the inside, leaving the historic fabric untouched. Planners almost always approve this.
| Comparison | Vacuum Glazing (Replacement) | Secondary Glazing (Addition) |
|---|---|---|
| Planning Risk | Medium (Requires Consent) | Very Low (Usually Permitted) |
| Visuals | Best (Looks like original) | Good (Discreet internal frame) |
| Soundproofing | Good | Excellent (Best Option) |
| Cost | High (Premium Product) | Medium (Cost Effective) |
6. Frequently Asked Questions
Yes, you need Listed Building Consent (LBC). Even though vacuum glazing looks like single glazing, it is a material alteration. However, precedent is growing for approvals across the UK.
The “sightline” is the width of the spacer bar inside the double glazed unit. If this is too thick, you can see the metal strip sticking out past the timber glazing bars, which ruins the historic look. Vacuum glazing eliminates this issue.
Yes, it is a premium product, costing significantly more than standard double glazing. However, when compared to the cost of moving house or the ongoing energy savings in a large heritage property, it is often a viable investment.
📚 Explore Our Heritage Glazing Hub
Discover more about preserving the character of your home while keeping the cold out:
Need expert advice for your Listed Building? Contact KJM today.
Mark Pearce is a highly experienced specialist in the double glazing industry, having built his knowledge from the ground up through hands-on experience in installation, manufacturing, sales, and business management. Beyond his role at KJM, Mark is a keen blogger, writer, and traveller, often sharing insights on the latest innovations in the home improvement industry.
- 2026 Design Trends: The 4 Window & Door Styles Defining the Year - 19 December 2025
- The 2026 Glazing Outlook” – High-level summary of the pivot to growth. - 9 December 2025
- Industry News: The Future Homes Standard 2025 & What It Means for Your Windows - 1 December 2025
