Renovation, replacement, or vertical extension? Assessing densification strategies to reduce the environmental impact

Renovating the existing building stock is crucial to reduce environmental impacts. Simultaneously, Switzerland, like many Global North countries, pursues urban densification. Although densification is often implemented through replacement construction, it remains unclear whether this strategy aligns with emissions mitigation targets. This paper investigates the impact of different densification and renovation strategies–replacement construction, renovation, and vertical extension (additions of extra floors to existing buildings)–on global warming potential (GWP). We apply life cycle assessment (LCA) to representative Swiss mid-rise multi-family housing archetypes and identify, based on global sensitivity analysis, the most effective decarbonization measures for low environmental impact intervention strategies. These strategies are evaluated and compared to conventional approaches under uncertainty from future component replacement, user behavior, decarbonization, and climate change. In agreement with previous studies, a fossil-free heating system and bio-based envelope retrofit are identified as the two most influential and robust measures to reduce GWP, i.e., independent of strategy, assumptions, and uncertainties. As demonstrated in practice, replacement construction can significantly reduce heating energy demand. However, suppose the goal is minimal GWP, vertical extensions and non-conventional renovations outperform replacement constructions: independent of the archetype, a conventional replacement construction results in 1.8 to 2.8 times the GWP of vertical extensions and exceeds Switzerland’s embodied carbon budget for the 1.5 ∘C target significantly. To support a socially sustainable shift to a low-carbon, densified building stock, soft densification strategies, like vertical extensions using bio-based materials, are crucial. While they may result in a more modest increase in built density compared to replacement constructions, they provide a more socially and environmentally sustainable method of urban densification.