Access issues and a minimal budget precluded the use of CLT for the long, low timber extension to a Grade-B listed Georgian terrace house in Edinburgh’s Bonnington Grove (2015), by Konishi Gaffney Architects.
Here the 36 metre long and relatively narrow 9 metre wide garden was not only urban, but provided unusual challenges. First amongst these was the non-statutory planning guidance that stated listed buildings in the city should have no more than 50% of their rear elevation built across, a problem in this case due to the fact that an existing, late-Victorian addition already occupied the west half of the facade. Agreement that the addition would not connect to the main house across the other half of the elevation threw up another hurdle: the extension would have to be sunk down into the ground to ensure that the resultant courtyard could not easily be glazed over to form an illicit conservatory, a requirement with inevitable, harmful, budget consequences.
In response, the architects chose to locate the addition against the garden wall that shadowed the site from the east and away from the house’s rear elevation but umbilically connected to it via the existing extension.
Cost considerations also led them to revisit the construction method developed in the 1960s by architect Walter Segal, a rigorous, but simple timber framing system that was gratefully adopted by self-builders then for its ease and economy and which now, with increasingly unaffordable property prices and pronounced housing shortages, is undergoing something of a renaissance.
Thus, a concrete and blockwork base was sunk one metre into the garden and tanked up to damp proof course level to form the plinth for a stick-built timber frame carefully set out at 600mm centres and expressed internally at the windows and in the joists below the ceiling of rough-sawn sarking boards. This being a kitchen, the boards have a coating of intumescent paint, whilst the walls and roof have 250mm of PUR insulation to compensate for the extent of glass relative to the extension’s floor area. Clever detailing of a trickle vent into the cladding cavity made the use of fixed windows possible and a benefit to the budget. All of the external timber has been painted, with the external plywood cladding fixed at the same centres as the glazing and detailed, in accordance with the principles of the Segal method, with cover strips to protect the joints. The expressed structure is accentuated by the building’s low-slung proportions and extended eaves which, together with its covered outdoor seating area, was achieved at the astonishingly low cost of £52,000.
Simple and affordable—and with more than a passing nod to the egalitarian genius of Walter Segal—the determined level of architectural intention that has underpinned the conception and delivery of this extension ably demonstrates that, when it comes to timber construction, clients should not presume that a restricted budget need result in a ‘cheap’ or low quality building.
It would be entirely wrong to describe this project as a ‘humble’ extension: Bonnington Grove is a model solution to a multiplicity of regulatory and technical challenges.
The Modern Timber House in the UK, chapter 8: The extended timber house