The environmental strain and depletion of natural resources caused by the current economic system could be reduced through the transition to a more circular economy, based on regenerative use of resources, where materials are kept in use for as long as possible and their value is retained (Cheshire, 2016; Ellen MacArthur Foundation, 2013; European Commission, 2015; Geldermans & Jacobson, 2015). The building industry has an important part to play in this transition, as it is a heavy user of raw materials and a large producer of construction and demolition waste (Merrild et al., 2016; Rijksoverheid, 2016). The circularity of the building industry can be improved by reducing waste production and preventing the need for additional material input by increasing the service life of existing structures and components (Platform CB’23, 2020; Rijksoverheid, 2018).  The service life of buildings and building components is usually limited by the frequently changing user demands and their inability to adapt to new requirements (Blok & van Herwijnen, 2005; Durmisevic, 2006; Landman, 2016). In Dutch residential buildings, the building services are often cast into the floors, creating one fixed element (Durmisevic, 2006; Landman, 2016). As a result of this integration, adaptation to changing technical requirements or different spatial topologies results in damage or even demolition of the load-bearing floor, reducing the overall service life of the floor system (Brand, 1994; Durmisevic, 2006). A way to remedy this is by designing the floor systems to be able to adapt to changing circumstances and user requirements through flexible design (Geraedts, 1996; Gijsbers, 2011; Schmidt et al., 2011). This way, flexible design could provide a practical approach to improve the circularity of building components such as floor systems. This leads to the main research question of this thesis: “To what extent can flexibility measures improve the circularity of prefabricated floor systems in residential buildings?” The main research question was addressed in two parts. First, the theoretical framework for the research was compiled, then the theory was put into a practical context through a case study. As part of the theoretical framework, a literature study was done into the factors that influence the flexibility of design, existing designs for flexible floor systems, and existing assessment methods for the circularity and flexibility of building components. In addition, interviews were held with people involved in different parts of the decision and design process of building projects to identify possible opportunities and challenges for the implementation of flexible design of floor systems.  As a result of the research into the factors that influence the flexibility of design, seven technical characteristics were identified that should be included in the design process to accomplish flexibility of design. These technical characteristics of flexible design are accessibility, durability, independence, integration, interface simplicity, overcapacity, and standardisation. Accessibility indicates the possibility to access components that need to be adapted without hindrance from other components. Durability regards the capacity of a material or system to continue to be useful after an extended period of time and usage. Independence concerns the technical separation of parts with different service lives and functions, while integration indicates the technical coupling of parts that are unlikely to ever be adapted separately. Interface simplicity requires the use of uncomplicated mechanical connections between elements to allow for easy disconnection. Overcapacity indicates the ability of a design to accommodate higher technical specifications than initially required and allows for flexibility in the application of the design in different functions. Finally, standardisation regards the use of the same components size and construction details throughout the design to ensure compatibility between different parts. From the interviews, it became apparent that flexible design provides opportunities for both the clients and the building industry itself. Flexible design offers clients the possibility to adapt their buildings to changing circumstances and requirements and plays into the growing interest and willingness of clients to invest in sustainable alternatives. It is expected that the opportunities regarding adaptability to future developments and residual value are especially interesting to professional investors. For the building industry, flexible design can have a positive impact on the efficiency of the collaboration between different disciplines in the design process. Other opportunities for the building industry lie with reduced failure costs and the possibility to increase the standardisation of building components. The main challenges for the adaptation of flexible design in practice, as identified through the interviews, are the conservative nature of the building industry and the possible cost increase of building projects as a result of the use of a novel floor system.  The literature study into the existing flexible floor designs showed that two types of flexible floor systems can be identified. The first type has the building services incorporated within the structural height of the floor and can be further subdivided into floor systems that are variations of the hollow core slab with grooves through which pipes can be laid and floors systems with a hollow space within the structural height of the floor for the placement of building services. The second type concerns systems where the building services are separated from the structural floor and the building services are either placed on top or below the load-bearing floor. Through the literature study, a variety of circularity assessment methods was identified. Based on the availability of the full method and the applicability on a building component level, the Building Circularity Indicator method was found to be the most relevant for this thesis. No suitable method was found for the flexibility assessment of building components. Therefore, a new flexibility assessment method was developed based on the seven technical characteristics of flexible design.  As part of the case study, the building code and stakeholder requirements for the design of a floor system were compiled and the existing flexible floor designs, as well as some traditionally used floor systems, were assessed on their circularity and flexibility to identify what characteristics of the floor designs contribute to their circularity and flexibility and where there was room for improvement. This was used as input for the development of a new flexible floor system. Several design concepts for flexible floor systems were worked out based on the incorporation of the technical characteristics of flexible design and with cost considerations in mind. The design concepts were developed, verified, adapted, and at times discarded, based on the building code and stakeholder requirements. The resulting designs are two versions of a prestressed ribbed concrete floor element. The ribs are placed at the top of the concrete slab. Between the ribs, there is space for the placement of building services and the ribs contain openings for the routing of pipes and cables. The gaps between the ribs are covered with panels, which remain removable for accessibility of the building services, and are filled with a loose granulate material for sound insulation.  The developed floor systems were assessed on their circularity and flexibility and compared to the existing flexible floor designs and traditional floor systems. This comparison showed that the developed floor systems have the highest flexibility score of all floor systems included in the comparison and also have circularity scores that are among the highest. The high circularity score as a result of a design based on flexibility and cost considerations indicates that the circularity of a floor system is improved by the inclusion of flexibility measures in the design. When looking at all the floor systems included in the comparison, a significant positive correlation between the circularity and flexibility scores of the floor systems was found. In conclusion, the implementation of flexibility measures can improve the circularity of prefabricated floor systems in residential buildings.