Koen van Velsen is the architect of a rich and diverse body of work spanning more than fifty years, ranging from cultural to residential, from healthcare to education, from infrastructure to offices, from urban design to buildings and landscape, and from new-build to restoration, renovation and transformation.

Each design by Koen van Velsen is a unique response to a specific location, programme, budget and particular requirements of users and clients. Spatial quality and the experience of the architecture are central to every project. Koen van Velsen values architectural craftsmanship and buildings that are carefully developed from concept to detail.

Although Koen van Velsen predominantly designed (large) public projects, his work is highly varied in scale, programme, character and complexity. All projects are the result of a specific design solution and are therefore devoid of a fixed preconceived architectural signature. Large or small, transformation or new-build, public or private: each project receives the attention it requires. Despite the diversity, projects by Koen van Velsen share important common characteristics such as 

the well-considered integration of buildings into the surrounding environment and a consistent focus on coherence, spatial quality and materialisation and detailling. All projects have a strong fundamental idea in common which resonates across all levels and during all project phases: from design to realisation. In this way, Koen van Velsen strives each and every time to finding the most appropriate response to each architectural question.

Between 2009 and 2015, Koen van Velsen was Dutch Railroad Architect on behalf of NS and ProRail. With a strong emphasis on an integral approach of design projects, Koen van Velsen strived for an improved cohesion between train stations and the surrounding public realm and the role and meaning of stations within this context.

In 2015, Koen van Velsen was acting Government Architect and a member of the Committee of Government Advisors. Koen van Velsen was also Chief Advisor for Spatial Quality & Design at Amsterdam Zuidasdok (2016–2024). In addition, Koen van Velsen has been jury member for the Abe Bonnema Architecture Prize, the Hedy d’Acona Prize, the Archiprix, the Europan and the BNA Cube.