Glass Box Extension

Crawford Partnership Architects were commissioned in October 2004 to completely renovate and extend the existing 2 storey 1930's cottage which is located within the Harpenden Conservation Area. Although significant in length, the narrow width of the cottage meant cramped hallways and small rooms with tiny windows and poor natural lighting. The outdated interior layout of this home was entirely at odds with its spacious and leafy surroundings.

In reviewing possible options for the new extension with our client it was eventually agreed to open up the back of the house virtually along its complete length and to provide a new single storey extension along the rear of the house to incorporate an open area that would link the proposed new kitchen, dining and family rooms. To the side of the house, the arrangement of out-house accommodation would be replaced with a new 2 storey extension. The whole upstairs would be overhauled and a new double height master bedroom would be created with an ensuite bathroom.

The Harpenden Conservation Area planning guidelines advise that all new extensions should be built in traditional materials that reflect those used within the main house construction; however, Crawford Partnership sought initial discussions with the Conservation Officer to propose a more contemporary design solution for the extension, with materials that included glass, steel and wood. The Conservation Officer indicated, to our surprise, that she might consider supporting a contemporary proposal (even though it would be the first time such a scheme would have received support in this area) but it was emphasised that this support would be subject to a very high standard of design being achieved.

Three options were offered for consideration, two of which were variations incorporating a solid flank wall and flat roof with continuous sliding folding glass doors opening onto the large south facing garden. Our own preferred third option was for an ‘all glass’ box incorporating silicone butt jointed full height glass panels and structural glass beams and columns.

We eventually received notification from the Conservation Officer that an ‘all glass’ box was the only option that would be considered and that this would be on the basis that no metal support whatsoever was visible as support structure.

On this basis a full planning permission was obtained for the design and subsequently as works on the main house commenced on site, and with soil analysis revealing that 2m+ foundations would be required for the new extension, it was agreed that a new basement should also be added into the equation. The basement would provide additional space for a gym or children’s play area and a utility room. It would also provide a solid box to build above ground works off.

Working with leading structural glass engineers, Malishev Wilson and the company Glasspace Ltd, the all-glass rear extension has been realised to create a space that adds a new dimension to the house, allowing rooms to be opened-up and flow between each other in contrast to the separation of activities caused by the narrow plan of the original house. Open-plan living has been achieved in a 1930s cottage.

The all-glass space allows for total interaction with the garden beyond, which was an integral part of the overall design for the property. Working with garden designer Paul Cooper, Paul has designed his garden to create vistas seen from specific locations within our design for the house. As the garden matures, planting will fill the space to the side of the glass-box further enhancing the feeling from within of being in touch with the garden beyond.

The all-glass rear extension is, we believe, the largest domestic example of its kind in the country and has only been achieved through recent advancements in glass technology. The frameless doors for example use a patented spider hinge fitting to allow for a totally frameless aesthetic. With its south facing location, a special transparent coating has been applied to the roof of the space to control solar gain into the space and alleviate the needs for additional blinds.

Aside from the rear all glass-extension, the basement also takes advantage of using glass technology with a walk-on toughened glass single-piece panel used to gain light into the below ground space whilst keeping the glazing flush with the ground floor level.

In terms of sustainability, the works have upgraded the original house into an energy-efficient dwelling. The single glazed windows and have all been replaced with new double-glazed low-e windows. The entire house has been insulated to meet modern standards, the existing lighting has been replaced with low energy fittings, the efficiency of the heating system has been vastly improved with a new condensing boiler and underfloor heating having been installed and the new basement provides buried in the ground provides an excellently thermally insulated space. The client is currently looking into the possibility of installing photo-voltaic panels on the south facing side of the roof – conservation area consent permitting.