The past two decades have seen phenomenal advances in building science: heat pumps that stabilize interior temperatures, photovoltaic panels that draw energy from the sky, automatic blinds set to computer algorithms that either maximize or minimize solar gain, depending on the season and time of day. If you kit out a building with enough state-of-the-art technology, you can — in theory — run it without consuming any gas or drawing any electricity from the grid.

Yet construction is still a dirty business. Most of the environmental costs of a given building come not from the energy it consumes but rather the materials it comprises. Glass, steel, concrete, wood — all are carbon-intensive. A new building has already done at least half the environmental damage it will ever do before anybody inside it flicks on a light switch or adjusts a heating dial. Its mere existence imposes an ecological cost.

To address this reality, we must build with the cleanest possible products — the ones that already clutter our landscape. This is the fundamental principle of circular design: a re-used material is always better than a recently manufactured one, and a retrofit or renovation — itself a form of recycling — is always preferable to a new build. What follows are four profiles of architectural projects from around the world, each with an original take on circular architecture and sustainability.

On an average day in London, England, municipal authorities fell 27 trees for reasons of safety or street maintenance, to make room for civic infrastructure or because the trees are diseased. Roughly five years ago, Je Ahn, founding director of local firm Studio Weave, found himself wondering where all the timber goes. So, over dinner, he asked a friend — an independent furniture-maker — who then asked a tree surgeon who works for the municipality. “The trees,” he learned, “end up in a yard, where they’re chipped and turned into mulch.” Could the wood be repurposed for architecture instead?

The Parks and Open Spaces Department in the London borough of Waltham Forest told Ahn that, if he wanted to use its timber in his publicly funded library project, he could have it for free as long as he could organize the logistics. This discovery became a key idea in exploring how to make his Lea Bridge Library, an addition to an Edwardian-era reading room in the neighbourhood, as sustainable as possible.

The addition itself is a rectangular, colonnaded extrusion — 250 square metres in size — that runs lengthwise along the back end of the original brick, stone and cast stone library. It has a café on one side and an event space on the other, all of it illuminated by floor-to-ceiling windows. Critically, Ahn’s design uses more than a dozen tree species, most salvaged from the city’s arboreal scrapyards. The beams and joists are pine or spruce glulam. The slats and panels are oak. The millwork is a many-hued amalgam of London plane, cherry, sycamore and lime.

The addition is similarly gracious toward individual trees that adorn the property. The foundations have lintel-capped holes, allowing large roots to pass through. Ahn left the final segment of the colonnade unbuilt to give space to a centuries-old London plane tree. At the centre of the structure — the line where the café becomes the meeting area — the exterior wall dips inward, forming a semicircular cut-out, so that the foundation might skirt the roots of two historic lime trees. The floors also jog upward at this point, so that an underground bunker, built during the Blitz and now encased in roots, can be left undisturbed.

This deference to trees — their location, their foundations, the colour and texture of their woody flesh — may seem eccentric, but Ahn insists that, among architects, it should be common sense. “The trees have been at this site, and in this city, for longer than I have, and they give people immense joy,” he says. “I could never design a building that’s better than a tree.

In 2016, the municipality of Meaford, Ontario, near Georgian Bay, brought on Brock James, a partner at the Toronto firm LGA Architectural Partners, to design a new public library. James studied several possible sites — the existing library, a local strip mall — but was most excited about one in particular: a boxy, stand-alone grocery store that had gone out of business. Why did he care so much about an unimaginative building that he describes as “plain” and “old”?

The answer has something to do with its floor size, 930 square metres, and also its location: at a prominent intersection, near the gateway to downtown. Another selling point was the building’s schematics. There’s nothing special about a concrete box, but it makes up in versatility what it lacks in novelty. “You can turn a box into nearly anything,” says James.

He outfitted the building with punched windows small enough that they don’t undermine lateral stability. To delineate interior space, he relied on millwork and ceiling details. You approach the new library at the north or south entrance, both of which are adorned with cedar slats, offsetting them from the concrete-plank facade.

Inside, there’s a checkout desk beneath a dropped ceiling. To the west, the ceiling rakes upward, inviting you into a well-furnished room where community activities happen in view of Sykes Street, the town’s main drag.

The stacks are in a 4.8-metre-high space to the east, which gets lower at the north and south ends, demarcating gathering areas for children and teens. The old grocery-store parking lot is now a green space that steps down to the Bighead River, where visitors fish for steelhead trout with rods on loan at the library desk. James concedes that the renovated building has “no overly precious details or architectural gymnastics,” yet, as a community hub, it does exactly what it’s meant to do.

You could make the same point about the structure it’s adapted from. Perhaps the best thing about the original grocery store is that, as George Mallory once said of Mount Everest, “it’s there.” If you want to build in high-density town centres, James argues — and you want to do so responsibly — you have to make use of pre-existing structures, and you can’t be too choosy. “Much of the carbon was already spent when the grocery store was first constructed,” he says. The most sustainable building is the one you don’t demolish.

When one hears the phrase “salvaged materials,” one often thinks of superficial accents: vintage floorboards, wood panels and fluted sconces, which can give a steak house or cocktail bar a pleasingly weathered look. But Building K.118 — a suite of think tanks and artists’ studios in the Swiss city of Winterthur, designed by the Zurich firm Baubüro In Situ — does something else entirely. Reclaimed elements constitute not just the finishes but also the structural core of the project. Architects Marc Angst, Pascal Hentschel and Benjamin Poignon gathered an extensive collection of used materials and thought deeply about each one: What does it ask of them? What opportunities does it afford?

K.118 is a four-storey block sitting atop a century-old brick warehouse, which is currently occupied by the Zurich University of Applied Sciences. Virtually everything in the addition came from elsewhere. The structural steel once supported an old distribution depot in Basel. The exterior cladding — sheet metal, tinted red to match the brick walls of the warehouse below — once adorned a print factory in Winterthur. And the exterior staircase, which serves as the main access point to the addition, was sourced from a Zurich office complex. The locations of the staircase landings dictated the heights of all three K.118 floors.

The addition’s interiors are similarly striking. Wardrobes and filing cabinets double as protective railings. Walls are lined with roof tiles from the surrounding cityscape. And the window casements — sourced from the same print factory that provided the exterior cladding — are insulated with the cheapest, most sustainable products going: straw bales and locally excavated clay.

Ultimately, Building K.118 is what people in the art world call a mixed-media work. Its conceptual forbears are not architectural projects so much as the sculptural assemblages by the likes of Marcel Duchamp, Joseph Cornell and Robert Rauschenberg — artists who breathed new life into ordinary materials, and who understood that the masterpieces of the future can be fashioned from the detritus of the past.

The AMP Centre, a skyscraper built in 1976 in Sydney, Australia, posed a problem for urbanists: It was too good to tear down but not good enough to keep. Lasse Lind, a partner at the Copenhagen firm 3XN/GXN, describes it as “a machine for people to sit in.” The tower, he explains, “was able to accommodate a lot of desks but didn’t have the open public or social spaces you’d expect from an office complex today.”

Yet it’s not as if the building had nothing going for it. The central core was sturdy. The ceilings were high. There were no load-bearing partition walls, so the interior space was flexible. “From an architectural standpoint, it was not up to date,” says Lind, “but it had good bones.”

So what to do with it? Implode it and send a perfectly serviceable tower to the scrapyards? Or renovate it, in which case you’re beholden to its many flaws? Lind and his peers decided on a third course of action. Beginning in 2019, workers under 3XN’s direction (BVN was executive architect) commenced two simultaneous projects at the site. They removed the cladding on the AMP Centre tower, stripping all facades down to the core. At the same time, they built a second tower right beside the deconstructed north face: a series of five stacked boxes, like a vertical village, each unit the size of a medium-rise building and each with its own multifloor atrium.

This new portion of the tower was built with temporary propping to allow it to stand on its own until it settled enough to be linked to the existing building. The last step was to connect the two structures, past and present, into an integrated whole, albeit with twice the capacity. The new skyscraper, called the Quay Quarter Tower, effectively cannibalizes the AMP Centre tower. To look at it, you wouldn’t know the old tower had ever been there — except that, in a meaningful sense, it’s still there. “We grafted a new building onto the existing one,” says Lind.

Because the Quay Quarter Tower was the first of its kind, the team faced daunting engineering challenges — and a fair bit of uncertainty. They had access to architectural proofs from the AMP Centre tower, but concrete buildings settle over time, and 50-year-old documents can only tell you so much. “You don’t know what’s behind the cladding,” says Fred Holt, a partner at 3XN/GXN, “and when you have a building that’s occupied, you can’t wait until the tenants are gone to find out if the project has re-use potential.”

So the team hedged its bets. As the partial demolition got underway, workers continuously took concrete samples to test the strength of the floor plates, and they outfitted the old structure with sensors to figure out how much the building was moving and whether the core was liable to collapse as the facade receded. When necessary, they modified their plans in real time, strengthening core walls and core link beams, as well as adjusting beam sizes and depths in the tower. This tactical approach paid off, resulting not only in a new building but also in a whole new kind of architecture — a rebuild–retrofit typology — that can be exported to other contexts. “We’re looking at another skyscraper in Sydney to see if it’s suitable for a similar restoration,” says Holt excitedly. “We’ve got a candidate in London too.”