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Shortlisted for New London Architecture’s Don’t Move, Improve award in 2024, Cairn Architects’ renovation and extension of a small Victorian terrace has a pioneering approach to materials

The construction industry is somewhat risk-averse, so if you want to use new or different materials to build with in the name of sustainability, it’s not always straightforward. But someone, somewhere has to pioneer new ideas – and in the case of this extension and renovation of a Victorian terrace in Hackney, emerging architecture practice Cairn stepped up.

Working with an environmentally conscious client, the project uses a new low-carbon concrete; hardwood instead of steel; and reclaimed materials or repurposed features as much as possible. The result is a project that uses 40% less CO2 than its more traditionally built equivalent – but aside from that, from an aesthetic point of view it also feels a more authentic reflection of the personalities of its owners.

Architect Kieran Hawkins, director of Cairn, says that his clients “live as environmentally soundly as they can, but they were they were nervous about doing work to their house because that’s intrinsically adding ‘stuff’ to the world, including carbon. They were very keen from the start to not have a concrete, steel and glass box: they wanted it to be low carbon and they also wanted to expose the natural materials wherever they could, because of the warmth and depth you get from that.”

Beyond the desire to tread lightly, the brief was to open up the ground floor to add space in the kitchen (one of the homeowners is a professional chef) and give more of a sense of flow between spaces; and bring in more natural light. With very little garden to extend into, Hawkins’ addition adds just 1.5m to the side, and the same to the rear of the house, while an existing flat-roofed area upstairs has been reinforced so it can be properly used as outside space, accessed via new French doors from a study.

The extension walls have a distinctive striated pattern, like layers of sand. “It’s hempcrete,” Hawkins explains. The hemp-based concrete substitute was poured layer by layer into plywood shuttering, with the architects, homeowners and builders all having a go (which gave rise to the project’s name: House Made By Many Hands). “Most people would render over the top, but we left it exposed. It has a great insulating and acoustic quality, and also regulates the humidity in the air. Hemp is one of those wonder materials,” says Hawkins.

The extension’s floor slab and underpinning uses limestone calcined clay cement (LC3 for short), a new material developed in Switzerland: Hawkins is the first in the UK to use it. “The high embodied carbon in concrete is largely down to how you make ordinary cement, which is by heating it to around 1500°C. LC3 uses calcined clay alongside a small amount of cement, and only needs heating to 800°C,” he explains. “One of the best things about it is that any builder can use it – I don’t think they even knew the difference.” It’s hard to underestimate the significance of this material: if universally adopted by the construction industry, it’s estimated that it has the capacity to reduce total global CO2 emissions by 1-2%.

While the LC3 is totally hidden, the other structural element, a beautiful frame made from the African hardwood sapele, skilfully jointed with flush dowels, definitely isn’t. When your structural materials don’t fit in with a pre-ordained list of what Building Control deems suitable, there will be extra hoops to jump through, as was the case here: “Sapele doesn’t have an official strength grading, so we had to get those specific timbers graded once they arrived on site. It was a cost and a hassle,” says Hawkins – but, like the LC3, the more people who take those first steps, the more it will become mainstream.

Aside from the pioneering use of materials, reuse is the major theme of this project. One of the redundant doors from downstairs has been turned into a pocket door for the upstairs loo, while the sink and some light fittings have also been reused. The floor that runs throughout most of the house was reclaimed from Bow Magistrates’ Court; kitchen appliances and the bathroom suite are second hand; and the kitchen worktops are made from Foresso, a terrazzo-like surface that uses waste wood chips (“it looks like nougat,” says Hawkins).

Having elements of the building that people can get involved in, even if they're not specialists, really pays back

Regarding that impressive figure of 40% less environmental impact, what surprised Hawkins the most was how much that was down to leaving materials exposed, like the lime-washed brickwork and the new hempcrete. No plasterboard, no lining and no paint all really added up.

His biggest takeaway, however, is how the act of everyone making the house together changed the nature of the project. “What I learned was how great it is to be able to make something together as a team. When we made the hempcrete and laid it – with us, the contractor and the client all chipping in – it really brought everyone together. There was just a different atmosphere on that project. Having elements of the building that people can get involved in, even if they’re not specialists, really pays back.”