Slim Double Glazed Sash Windows

The Conservation Area Problem

You want double glazing for warmth, noise reduction, and lower energy bills. Your Conservation Officer wants windows that look exactly like the originals — which means single glazing with slim timber sections. Standard double glazed units at 24–28mm are too thick for original sash profiles and require wider rails that immediately look wrong on a period property. This conflict has stalled thousands of window projects across London.

Slim heritage double glazed units solve it. At 14mm total thickness (4mm Low-E + 6mm argon cavity + 4mm float), they fit inside the same sash depth as original single glazing. The sash rails stay slim. The sightlines stay authentic. And you get genuine double glazing performance.

14mm vs 24mm: Honest Comparison

When to Choose Slim Heritage Units

Choose 14mm slim units when your property is in a Conservation Area and the planning authority requires minimal visual change, when you are replacing sashes in an existing box frame and the rebate depth is limited, when the aesthetic of slim sash rails is important to you, or when you have a Listed Building and any alteration to the window proportions would not be permitted.

When Standard 24mm Is Better

If your property is not in a Conservation Area and appearance is secondary to thermal performance, standard 24mm units are the better choice. They deliver a U-value of 1.2 W/m²K vs 1.8 for slim units — a meaningful improvement in heat retention and energy savings. Our 3D configurator lets you compare both options on your exact window dimensions with instant pricing.

Planning Permission

Slim heritage units are now widely accepted by Conservation Officers across London boroughs. In most cases, a well-specified timber sash window with 14mm heritage glazing will receive approval without objection. We can provide technical drawings and specifications to support your application — contact our team for assistance with the planning process.