The OWO: Modern glazing for heritage joinery

The Old War Office (OWO) in Whitehall has undergone a significant transformation, converting one of London’s most recognisable historic buildings into a luxury hotel and residential development. As a Grade II* listed structure, the project demanded a careful balance between preserving architectural character and meeting modern performance standards.

A key element of this transformation was the window strategy. Rather than restoring existing frames, new timber sash windows were introduced, designed to replicate the original proportions, sightlines and detailing of the Edwardian façade. This approach ensured the building retained its historic appearance while allowing for improved durability and performance.

With large window openings across a busy central London location, acoustic and thermal performance were critical considerations. The building is exposed to constant traffic noise, and the expectations of a high-end hotel and residential environment require a significantly higher level of comfort than the building’s original use.

Crystal Units supplied a glazing solution based on laminated and low-emissivity insulating glass, designed to enhance sound reduction, thermal efficiency and safety. The challenge was to achieve these performance improvements without altering the visual integrity of the new timber joinery.

The scale of the windows added further complexity. Oversized sash windows meant increased glass weights, placing restrictions on what could be achieved within the vertical sliding system. Meeting acoustic targets while working within these structural limitations required careful coordination between glazing specification and joinery design.

Throughout the project, the glazing was considered as part of the overall window system rather than a separate element. This integrated approach ensured that performance requirements were met without compromising the architectural intent.

Phil Hallworth, Technical and Sales Director at Crystal Units, commented: “On a building like The OWO, you don’t get much room for error. Everything is visible, and if something’s out, it shows straight away.

“Even though the windows are new, they still had to carry the same visual weight as the originals. That meant the glass had to work with the joinery, not against it. At the same time, we were dealing with a completely different set of expectations in terms of acoustics and thermal performance.

“The size of the windows made it more challenging as well. Once you’re working with that kind of scale and weight, you can’t rely on standard approaches. It all has to be thought through together, from the frame to the balance to the glass itself.”

Closing

The OWO highlights how modern glazing can support new joinery within heritage settings when both are designed in tandem. By aligning performance requirements with architectural detail, the project demonstrates how historic character can be retained while delivering the standards expected of contemporary buildings.

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