Integrating occupancy density into the environmental assessment of residential buildings: Towards embodied impact reduction at both building and urban level

Life Cycle Assessments (LCA) of buildings typically use gross floor area (GFA)-centric functional units, while urban-scale assessments use occupancy-centric ones. This mismatch reflects the dual functions of buildings (providing space and shelter) and can lead to conflicting strategies, where reducing impacts per GFA at the building level may increase impacts per occupant at the urban level. This study compares these assessment approaches and introduces a dual-functional unit approach that evaluates buildings across both functions. Eight detached houses are assessed using LCA across four impact categories and ranked using a multi-criteria decision-making (MCDM) method for each approach. Results show significant variation in rankings among the established approaches, with several buildings having completely opposite outcomes (e.g., ranked 1st in the GFA-centric assessment but 6th in the occupancy-centric assessment), while the proposed approach produces more consistent rankings, representing both functions. Moreover, a scenario-based analysis compares the three assessments to a control (no-assessment) scenario, considering all detached houses built in Aotearoa New Zealand over the past 5 years. The GFA-centric assessment resulted in increased total impacts (mean difference +0.20% across the four impact categories) by promoting larger and less dense buildings (+0.70 m² per occupant), while the occupancy-centric assessment led to mean impact reductions of -1.98%, while significantly reducing the space per occupant by 0.98m². The proposed approach achieved even greater impact reductions (-2.22%) while reducing space by only 0.31m² per occupant. Finally, a correlation with national climate goals was made, showing the approach could achieve 71.65% of the national carbon reduction target.