Key Takeaways
- Gowercroft supplied bespoke Accoya timber heritage windows for an extension to Grade II* listed Holbrook Hall
- The windows met strict planning constraints while improving thermal performance
- A combination of flush casement and sliding sash windows was used, with slimline double glazing to replicate the look of single glazing
- Planning consent was achieved after Gowercroft provided samples showing no visible difference between slimline and single-glazed options
- The project was a collaboration between Gowercroft and award-winning architect Darren Mayner, with full satisfaction from the client and conservation officer
How did this project at Holbrook Hall begin?
In 2011, Holbrook Hall in Derbyshire, a Grade II listed building* and residential care home, underwent a sensitive extension. The project was designed and project managed by Darren Mayner BAHons BArch ARB, Principal Architect at Darren Mayner Architecture.
Darren knew the extension would require bespoke timber windows that carefully respected the existing architecture. Gowercroft worked closely with him to deliver a highly specialist solution: one that balanced historic aesthetics with modern performance.
Why was Accoya chosen for the heritage windows?
“When I designed the recently completed extension to Holbrook Hall, a Grade II* listed building, I knew that bespoke timber windows would be required. I designed two types of window for this project: flush casement and counterbalanced sliding sash. Both types were designed to match the details of existing windows and required very thin glazing bars.”
— Darren Mayner, Architect
The original windows were all single-glazed, and the planning authority initially required that all new windows should match.
Their concern? That visible spacer bars and double reflections from modern glazing would spoil the look of the listed building.
Many of the existing windows had secondary glazing to improve energy efficiency, but this wasn’t an ideal solution for the new build. Darren sought a better option.
He was already aware of two specialist products:
- Slim double-glazing, which could fit into narrow glazing bars
- Accoya timber, which provides long life with enhanced stability and sustainability
Gowercroft offered both.
What made Gowercroft the right choice?
“I found that Gowercroft offered both the slim double-glazing and Accoya timber products that I required for this project, and when I contacted them Andy was happy to give me a guided tour of their small factory, still in Wirksworth at the time. He then provided quotes for different materials: softwood, hardwood and Accoya. The work I saw was of a high standard and they were friendly and helpful. It seems to be run like a small family business with customer care being a high priority!”
— Darren Mayner, Architect
Darren appointed Gowercroft after visiting the workshop. He was impressed not only by the quality of the work, but also by the team’s openness and commitment to customer care.
How did Gowercroft help secure planning approval?
Once the contract was agreed, Andy and the Gowercroft team supplied detailed timber profiles based on Darren’s drawings. They also produced a fully finished window sample for approval by the local conservation officer.
“At my request, this sample was glazed partly with single-glazing and partly with slim double-glazing and was very helpful in convincing the Conservation Officer to remove the single-glazing planning condition, as no discernible difference could be seen between the two glazing types.”
— Darren Mayner, Architect
This step proved crucial. Without it, the extension may have been forced to use outdated glazing methods that compromised performance.
What additional support did Gowercroft provide?
Gowercroft helped specify ironmongery for the project, sourcing specialist components and providing samples for approval. Once installed, all double-glazed windows were factory finished and fitted by Gowercroft to the satisfaction of the architect, the client, and the conservation officer at Amber Valley Borough Council.
What has changed since the Holbrook Hall project?
Much has changed since 2011, both in glazing technology and at Gowercroft Joinery.
At the time, slimline double glazing was one of the few options available for improving thermal performance in listed and heritage buildings. Although it could fit into narrow glazing bars, it came with several drawbacks: shorter lifespan, visible spacer bars, and a high failure rate over time.
Today, we’ve replaced slimline glazing with a more advanced solution. After years of development and testing, Gowercroft now uses vacuum glazing in our heritage window and door ranges. It offers significantly better performance, with U-values as low as 0.4 W/m²K, excellent clarity, and the ability to replicate the appearance of traditional single glazing.
This change reflects one of our core values: always using the best technology for each application. We continue to refine our materials, coatings, and glazing systems to deliver outstanding results. For every product, our goal is to combine longevity, performance, and authenticity, especially in sensitive conservation settings.
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