Heritage and listed-building work lives or dies on sightlines, authenticity and what the conservation officer will accept. Specifying a unit that is too thick, has the wrong sightline or looks modern is one of the quickest ways to get a rejection and delay the programme.
At The Glass Shop Online we supply slimline double glazed units and vacuum units for period properties, sash windows and listed buildings. We help trade customers choose options that improve thermal performance while staying as close as possible to original appearance.
Slimmest Units That Still Get Used
The units most commonly used on heritage-type buildings are:
- Standard slimline DGUs — 12 mm or 14 mm overall thickness
- Vacuum units — around 8 mm overall
Whether a given thickness is accepted is decided by the building, the local authority and the conservation officer. There is no single national “approved” thickness that works everywhere.
When to Recommend Slimline vs Vacuum
Slimline units offer clear thermal and UV performance gains over single glazing in a relatively thin package. They are the practical first choice when replacing old single glazing — provided the existing frames can take the thickness and planning allows the change.
Vacuum units are the route when the frame or the conservation officer will not accept even slimline thickness. They are significantly more expensive, so they are usually reserved for the cases where slimline is ruled out.
Common Reasons Conservation Officers Reject Applications
Typical rejection grounds include:
- The proposal is not consistent with the period property or neighbouring buildings
- Modern frames or non-authentic materials
- Sightlines that are too large for the frames or that look contemporary
Keeping sightlines tight and the overall appearance close to the original is essential.
Typical Sightline Requirements
For listed buildings, maximum sightlines are commonly in the region of 5.5 mm to 7 mm. Always confirm the exact expectation for the specific property with the conservation officer or planning consultant before ordering glass.
What Helps Approvals
Clear product data helps: overall thickness, sightline dimensions, thermal and UV performance figures. We can supply data sheets with the glass so you have something concrete to put in front of the conservation officer. Final acceptance still rests with the local authority.
What We Supply for Heritage Work
| Option | Typical Overall Thickness | Notes for Trade |
|---|---|---|
| Slimline DGU | 12 mm or 14 mm | First choice when frame and planning allow. Better thermal/UV than single glazing. |
| Vacuum unit | ~8 mm | When slimline is rejected on thickness. Higher cost. |
| Sightlines | Target 5.5–7 mm max | Critical for listed work. Confirm local requirements. |
How We Support Heritage & Joinery Trade
We supply slimline and vacuum units cut to the sizes your frames need. Send us the required overall thickness, sightline targets and any performance data the conservation officer has asked for. We will match a practical option and provide the documentation to support the application.
Send us the required overall thickness, sightline targets and any performance data requested by the conservation officer. We will confirm a workable specification and provide a trade quote.
Questions heritage specialists still ask
What is the thinnest unit you can supply?
Standard slimline units are typically 12 mm or 14 mm overall. Vacuum units around 8 mm are available where the thinner profile is essential. Acceptance always depends on the individual building and the conservation officer.
When should I specify vacuum instead of slimline?
When the existing frames or the conservation officer will not accept even slimline thickness. Vacuum is thinner but more expensive, so it is usually the fallback option.
What sightline should I aim for on listed work?
Common maximums are around 5.5 mm to 7 mm. Confirm the exact requirement for the property before ordering.
What paperwork helps with approvals?
Data sheets showing overall thickness, sightlines and thermal/UV performance. We can supply these with the glass. Final decision still sits with the local authority.