Background and relevance

Danish and Dutch high-density cities such as Copenhagen and Amsterdam face similar challenges : growing demand for housing, scarce and expensive building land, and the need to develop more sustainably. Vertical extension presents a partial solution: adding one or more storeys on top of an existing building to create new homes. This approach has gained significant momentum in the Netherlands over recent years, driven by housing shortages, sustainability ambitions, and advances in lightweight prefabricated construction. Copenhagen shares many of the same urban conditions as Amsterdam: dense inner-city neighbourhoods, extensive post-war apartment stock, bicycle-friendly streets, and comparable ownership structures. Therefore, this project explored whether and how the Dutch experience can be translated to the Copenhagen context. In addition, our partners explored in more detail which buildings are suitable for vertical extension in Copenhagen and what design solutions would be fitting from both a technical, economic and social perspective. 

Research question: ‍

What is the potential for vertical extension in Copenhagen, and what conditions, examples, and lessons from the Netherlands are most relevant for Danish building owners, architects, developers, and local governments?

Method: 

The research combined a spatial analysis of Copenhagen's rooftop potential with a review of Dutch case studies, existing research, and best-practice documentation. This resulted in an inspiration guide in English, drawing on interviews with Dutch architects, developers, and intermediary parties, and presenting real-life examples with relevant lessons for a Danish audience. Our partner Tegnestuen LOKAL conducted an extensive design study to explore what typologies would be suitable in Copenhagen. A joint report in Danish was produced together with the project partners that combines all these findings.

Key insights:

• Vertical extension proves to be economically competitive with conventional new-build in dense, high-value urban areas, where the elimination of land costs compensates for the higher construction complexity both technically and socially. 
• The business case becomes even stronger when vertical extension is combined with planned roof renovation and sustainability upgrades.
• Common challenges include resident concerns, parking pressure, regulatory misalignment, and technical complexity around installations. However, all of these can be addressed with the right approach and early stakeholder engagement.
• Lightweight, prefabricated, biobased construction methods are advancing rapidly, reducing costs, construction time, and environmental impact.

Recommendations:

• Building owners (from private home-owner associations to (social) housing associations and institutional investors) should assess their rooftop potential and investigate combining vertical extension with planned renovations and sustainability improvements, involving residents from the very start while doing so. 
• Architects and developers should build on the growing body of technical knowledge and guidelines, and embrace prefabricated biobased construction methods to improve the business model.
• Local governments should integrate vertical extension into zoning plans for post-war neighbourhoods, review parking norms that block the creation of new homes, and establish cross-departmental coordination, following the example of cities like Amsterdam and Rotterdam.

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