Vertical Extensions; From Incidental to Fundamental

Abstract

The Netherlands faces a persistent housing shortage alongside strict spatial and environmental constraints, which limit greenfield development and increase pressure on inner-city densification. Vertical extensions, defined as the addition of one or more residential storeys to an existing housing structure, offer a promising form of densification, yet their realisation remains largely incidental. Existing research has focused on technical feasibility and sustainability impacts, while the conditions required for scaling vertical extensions within the housing regime are underexplored. This thesis aims to identify the key conditions for scaling vertical extensions in the Dutch housing system and to translate these into a transition strategy. Scaling is defined here not as the replication of a product, but as the integration of vertical extensions into broader societal and institutional contexts. A qualitative multiple-case study design is employed, examining four pilot municipalities. Data is collected through semi-structured expert interviews and document analysis and a literature review. The analysis combines the Multi-Level Perspective and the X-curve to interpret the transition dynamics of vertical extensions in the Netherlands. Vertical extension pilots are positioned as a series of transition experiments that inform scaling through niche build-up and regime transformation. The findings show that governance, financial and social conditions determine whether vertical extensions can be deployed at scale and are strongly context dependent. By contrast, technical solutions for vertical extensions and industrialised construction are found to be comparatively transferable between cases and do not appear to constitute the primary bottleneck. Key enabling conditions include political leadership, clear municipal coordination, opportunities to combine vertical extensions with renovation and other interventions to realise economies of scale, supportive land and parking policies and cross-organisational learning. Barriers arise from institutional caution, misaligned cost allocation, land-value capture mechanisms and conflicting municipal policy objectives. The thesis develops a transition strategy that sequences interventions along the X-curve, linking niche build-up to regime reconfiguration. The study contributes to transition theory by empirically illustrating how niche dynamics and stakeholder influence evolve, while offering practitioners a phased strategy to scale vertical extensions from incidental to more systemic forms of inner-city densification as a contribution to the national housing shortage.