This study explores how rooftop extension projects can be evaluated more systematically, paying specific attention to their strategic, technical, financial, and societal implications within portfolio-based investment decisions. In a context of mounting housing shortages and spatial constraints in Dutch cities, innovative densification strategies are becoming increasingly important. Rooftop extensions, adding extra layers to existing buildings, are often regarded as promising because they add dwellings without additional land take and align with circular ambitions. In practice, however, housing associations still treat such projects largely as incidental initiatives, assessed with generic tools originally designed for new-build or large-scale renovation. The research combines a literature review on adaptive reuse and investment structures with five case studies. For each case, two interviews were conducted, one at a strategic level and one at an operational level, and the conceptual assessment framework was validated by an expert panel. The findings show that decision making on better use of the existing stock is evolving in some associations. Risk perceptions, such as technical uncertainties, planning constraints, or construction in occupied buildings, are examples of identified barriers. At the same time, the cases illustrate that rooftop extensions can support integrated renovation, enhance neighborhood livability, and increase typological diversity.
To help housing associations assess these projects more systematically and enables to argue investment decisions more robustly, an evaluation framework was developed with three components:
• Knock-out criteria, such as critical project mass;
• Opportunity valuation, for aspects such as circularity and housing differentiation;
• Key considerations relating to risks and policy alignment.
The study concludes that further professionalization is needed to embed rooftop extensions firmly within development and investment strategies. This requires explicit integration into portfolio policies, greater attention to societal value, early coordination with municipalities, and ongoing internal knowledge development. Standardizing design and permitting procedures can facilitate wider applicability and scaling.