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Old Mullions is a frameless glass extension to a Grade II listed 17th-century stone house in Honington, Warwickshire. The extension replaced a deteriorating 1970s lean-to structure with a contemporary, transparent pavilion housing a new kitchen and dining space.

The project demonstrates how minimal glazing systems can be integrated with historic stonework to meet conservation area requirements while maximising natural light in a north-facing extension. IQ Glass designed, manufactured, and installed the complete glazing package, including an 8-metre frameless strip rooflight and bi-parting slim sliding doors with 21mm sightlines.

Products Used

Project Partners

Architects: Hayward Smart Architects

Contractor: Envy Brick Work

Image credits: Richard Chivers

Location

Warwickshire

Fixed strip rooflight to modern glazed extension

Lantern-like modern rear glass box extension

Planning Constraints & Heritage Considerations

The rear elevation had been extended over time with a poorly constructed 1970s lean-to that offered inadequate connections to the garden and minimal natural light.

The homeowners sought to replace this structure with a modern extension that would provide a functional kitchen and dining space while respecting the historic character of the listed building.

The building's location within a conservation area and its Grade II listed status required careful consideration during the design and planning approval process. The key planning requirements were:

1. Visual transparency – The extension must appear lightweight and transparent to avoid obscuring the historic stonework
2. Minimal visual impact – The design must "touch the house lightly" and avoid competing with the original building's architectural character
3. Heritage visibility – The original stone walls must remain visible through the glazing from both inside and outside
4. North-facing orientation – The extension faces north, requiring careful design to maximise light ingress without relying on solar gain

Hayward Smart Architects consulted IQ Glass during the early design phase to establish what was technically feasible with a frameless glazing system and to obtain specification details that would support a successful planning application.

Planning approval was granted on the basis that the extension would be fully transparent, structurally minimal, and would preserve views of the original stonework.

IQ Glass was contracted to design, manufacture, and install the complete glazing package for this Grade II listed extension. The primary technical challenge was achieving a fully frameless appearance while meeting structural and thermal performance requirements.

The rooflight design required an 8-metre continuous span of structural glass without visible framing. IQ Glass specified a silicone-bonded Invisio strip rooflight and slim sliding glass doors to create a bright airy space with seamless transitions to the garden.

The Invisio frameless glass strip rooflight spans just under 8m across the new glazed extension to this Grade II listed Warwickshire home consisting of four fixed panels each weighing 133kg silicone bonded for the ultimate minimal finish offering uninterrupted views of the sky whilst allowing balanced natural light to flood into the internal space. The strip rooflight uses 8mm toughened glass.

On the garden-facing elevations, IQ Glass installed bi-parting slim sliding glass doors with 21mm sightlines. These doors were specified to align structurally with the extension's timber beams, creating a symmetrical composition that responds to the building's geometry.

The sliding door system was thermally broken and bonded with glass to achieve minimal frame visibility. The doors provide full-height glazing from floor to ceiling, maximising views to the garden and creating direct access to the outdoor terrace.

A critical design detail was the integration between the new glazing and the existing historic stonework. IQ Glass worked closely with the architect and contractor to develop bespoke metal channels and fixing details that were set into the stonework. These fixings create a completely frameless junction where the glass meets the stone, ensuring that the historic wall remains fully visible and unobstructed.

The original timber beam within the ceiling was retained and exposed, providing continuity with the building's heritage character while supporting the new glazing structure.

Flush thresholds were specified for all sliding doors, providing barrier-free access between the internal kitchen space and the external patio. This detail was designed to meet accessibility standards while maintaining a clean visual transition.

Flush threshold base rails to Warwickshire glass extension

Bi parting sliding doors to Warwickshire glass box extension

Frameless glass extension to Grade II listed Old Mullions House

Thermal Performance & Natural Light Management

As the extension faces north, there was minimal risk of overheating from solar gain. However, thermal performance remained a priority given the extension's role as a primary living space during winter months.

All glazing was specified as double-glazed units with low-emissivity (low-E) coatings and argon gas fill. This specification provides a U-value suitable for year-round thermal comfort while maintaining high levels of visible light transmission.

The 8-metre strip rooflight was designed to distribute natural light evenly across the depth of the extension. Because the extension is single-storey and north-facing, the rooflight serves as the primary source of daylight penetration, preventing the space from feeling enclosed or dark.

The thermal performance of the rooflight was carefully specified to balance insulation with light ingress. The 8mm toughened glass panels provide structural integrity while the silicone-bonded joints allow for thermal movement without compromising weathertightness.

Extension Dimensions:
- Overall extension footprint: Single-storey pavilion
- Strip rooflight span: 8 metres
- Rooflight panel configuration: Four fixed panels, silicone-bonded
- Individual panel weight: 133kg each
- Rooflight glass specification: 8mm toughened safety glass

Sliding Door System:
- System type: Thermally broken aluminium with bonded glass
- Sightlines: 21mm
- Door configuration: Bi-parting (two panels slide in opposite directions)
- Threshold: Flush, barrier-free access
- Glazing: Double-glazed units with low-E coating and argon gas fill

Thermal Performance:
- All glazing: Low-E coated double-glazed units with argon fill
- U-value: Suitable for year-round thermal comfort (specific value dependent on glass build-up)

Finishes:
- Timber cassette detail around rooflight perimeter
- Bespoke metal fixing channels set into historic stonework
- Retained original timber ceiling beam
- Timber cladding to soffit extending over entrance door

The project demonstrates that it is possible to introduce modern glazing systems into historic settings without compromising the character of the original building. The key to success is early consultation with specialist glazing contractors to establish technical feasibility and to ensure that planning submissions are supported by accurate specifications and performance data.

Are you working on a Listed Building requiring a modern glass box extension compliant with Building Regulations?

Contact the team today! hello@iqglassuk.com