Unpacking suitability for building vertical extensions: a multi-level clustering analysis of factors

Vertical Extension (VE) of buildings offers promising pathways for sustainable urban densification. This study conducts the most comprehensive systematic literature review to date on VE suitability, following PRISMA guidelines and using content coding in ATLAS.ti combined with co-occurrence analysis to ensure methodological transparency and reproducibility. A total of 119 publications were analyzed, and 88 suitability factors were synthesized. The research provides a systematic knowledge of multidimensional considerations in VE implementation by thematically classifying the factors, mapping them across three suitability levels – site, building, and design – and assessing temporal/geographic patterns. The findings indicate a pronounced emphasis on technical and structural aspects, especially at the building level, whereas softer features like community engagement and multi-scalar governance are inadequately examined. Site-level factors are significantly marginalized, which may restrict the scalability and contextual adaptability of VE strategies. The novelty of this work lies in providing a multi-level clustering framework that not only maps the distribution of factors but also critically interprets research gaps, offering an integrated reference for architects, engineers, planners, and policymakers. While the study is based on literature analysis, it establishes a transparent, reproducible basis that future case studies and multi-criteria decision models can build upon to achieve more holistic VE assessments.