A seven-storey cross-laminated timber extension built on top of a 1950s London concrete frame has saved more than 4,500 tonnes of embodied carbon and increased the building’s total floor area by 40 per cent — the latest example of vertical extensions, an emerging building typology where developers take advantage of lightweight engineered wood products to build new structures atop steel and concrete assets.
That is according to Fred Schwass, chief development officer at General Projects, who spoke to Forbes about his Metropolis scheme, a project that retained the original structure of the former Woolworths headquarters before adding a CLT upper extension that delivered a BREEAM Outstanding rating and an operationally net-zero outcome, powered by renewable energy.
“There is a strong demand and interest for this type of sustainable technology,” Schwass said, adding that interest is gaining traction across Europe as a practical pathway to make older concrete buildings fit for the demands of the 21st century.
Vertical extensions work because CLT and glulam are lightweight — a structural reality that, as Wood Central reported last year, has already seen the typology gain traction in Milan ahead of the Winter Olympics, with Italian-based Rubner Holzbau Srl completing a vertical timber addition to accommodate spectator overflow for the Milan-Cortina Games.
Speaking to Wood Central last year, Gianluigi Traetta, Technical Sales Engineer for Rubner Holzbau Srl and a Partner Investigator at Australia’s ARC Advance Timber Hub, said the material’s weight makes it viable where heavier systems are not.
“Timber is perfect for these projects,” Traetta said, adding that lightweight timber systems can help developers effectively double the floor area of existing assets. “The most important advantage is its lightness. In most cases, we do not need to reinforce the foundations. Sometimes, we have to work in very constrained areas and be as quick as possible because you bring disruption to the people living there and so on — so the fast-paced nature of construction is also a major advantage.”
Charlie Law, sustainability director at Timber Development UK, said extensions allow asset managers to utilise existing structures, eliminating the embodied carbon cost of starting from scratch. And at the same time, the materials profile makes upper-level additions structurally viable where concrete or steel would trigger costly foundation work.
“By keeping an existing concrete building in place, but increasing its usable floor area by the addition of a timber extension, either upwards or outwards, we are avoiding the waste of embodied carbon emissions of demolishing the existing building and having to build new,” he said.
It’s lighter, cleaner, safer and has much less disruption
Manufactured off-site, Law said extensions can reduce disruption in urban environments and can also compress construction timelines relative to in-situ alternatives. And demand is not being driven solely by carbon targets.
Richard Meacham, an architect and design integration director at DIRTT, said hybrid timber buildings tap into biophilic design principles — and, increasingly, into the corporate urgency of getting workers back into city offices.
“Everybody loves wood, and it’s a material we just naturally connect with,” Meacham said. “We are seeing a growing trend for sustainability, and also the need to make spaces more attractive, because businesses are trying to get people back in the office, and maybe differentiate themselves as a company.”
Meanwhile, Schwass said any building can be given new life without demolition: “I think the norm going forward will be more and more hybrid buildings, where you have cross-laminated timber slabs with steel frames.”
- To learn more about vertical extensions, click here for Wood Central’s special feature — Think Vertical! Extensions Could Solve Brisbane’s Olympic Puzzle — examines how the typology could help cities rapidly scale accommodation ahead of major events, including the 2032 Games.
