K118, located in Winterthur, Switzerland, is a pioneering project that transformed a former carpentry factory into a mixed-use building for workshops and think tank offices. Completed in 2021, the renovation and expansion leveraged circular construction principles, including reuse, repair, refurbishment, repurposing, and recycling.

The construction industry faces challenges balancing sustainability, cost-efficiency, and regulatory compliance. Traditional practices often rely on virgin materials, contributing to significant environmental impacts, particularly greenhouse gas emissions.

Renovation projects like K118 further complicate these challenges, requiring integrating new and reused materials while maintaining structural integrity, safety, and functionality.

Additionally, there is a lack of established frameworks for incorporating circularity principles into construction, leaving architects and engineers to navigate complex trade-offs and logistical hurdles.

The K118 project adopted a multi-layered approach based on circular construction strategies:

- Material Reuse and Repurpose: Key components such as steel staircases, windows, roof elements, and granite slabs were sourced from local building demolitions. This reduced the reliance on virgin materials while maintaining structural integrity.

- Repair and Refurbishment: Over 2,700 components were assessed for reuse, with necessary adjustments to meet modern safety and functional standards. For example, banisters on a reused steel staircase were retrofitted.

- Recycle Where Necessary: Recycled alternatives were prioritised for materials unsuitable for reuse, including in concrete applications.

- Architect-Led Processes: Architects reversed conventional workflows by inventorying available materials before finalising the design, integrating sourcing logistics, and collaborating with interdisciplinary teams to optimise material circularity.

The project achieved a 41% circular (non-virgin) material use rate in volume, avoiding 494 tonnes of CO2eq during construction. This corresponds to a 59% reduction or avoidance in construction-phase emissions compared to a building made with virgin materials.

Dismantling and reassembly tend to increase costs, particularly in Switzerland. As a result, upfront costs were higher than in conventional construction, with 11% of the total cost incurred before construction began. However, in the end, the project was only 2.5% over budget (compared to the initial estimate), showing that reuse can be financially competitive with conventional methods when carefully planned.

In addition to environmental and financial benefits, the K.118 project also serves as a proof of concept for the potential for circular construction to strengthen the local economy by creating new skilled jobs. 

The project highlights emerging opportunities for architects to expand their traditional role and scope within circular construction and renovation projects. In particular, K.118 highlights architects' new roles in interdisciplinary collaboration, logistics optimisation during component sourcing, and oversight of component reuse processes.