This research investigated how traditional foundations of buildings can become circular and thus contribute to a circular economy. The current linear take-make-dispose economy results in waste, exhaustion of finite natural resources and environmental degradation. A circular economy is one way to cope with these trends. This type of economy is based on closed material cycles, renewable energy and system thinking. For the building industry, recycling of materials and reuse of products are most relevant. Reuse of products is preferred because it requires less material and energy than recycling. Reuse is facilitated by flexibility. Forms of flexibility are versatility, changeability and expandability. For building foundations, versatility and changeability are distinguished. Expandability should be incorporated in both. Within the circular economy, many design methods have been developed. The tools Design for Disassembly and Design for Adaptability help to design buildings for changeability and versatility, respectively. Corresponding assessment methods have been developed. An easy-to-use method suitable for the assessment of building foundations was sought and found: Alba Concept. Four foundation variants were defined by combining deep and shallow foundations with versatility and changeability. Based on the soil condition, loads, future scenario, function and desired level of flexibility, the foundation type can be determined. Hereafter, a design guideline can be followed, considering material, dimension, load bearing capacity, connection and transportation. One should strive for longevity, standardization and uniformity at the material, element and system levels, respectively. Because the assessment method of Alba Concepts focuses on changeability and buildings in general, it was modified. This method focuses on circular foundations in particular and values changeable and versatile designs. To determine whether the theoretical concept of circular foundations leads to foundations that are more circular than the traditional ones, two case studies were conducted. The considered buildings required a deep and shallow foundation. For both projects, a changeable and versatile foundation was designed. All foundations were assessed by the Alba Concepts’ method and the alternative method. Independent of the assessment method, the traditional foundations have a low level of circularity. Because Alba Concepts focuses on changeability, the versatile and changeable foundations obtained a low and high score, respectively. Based on the alternative method, all circular foundations obtained a high level of circularity. Finally, the structural similarities, differences, challenges, preconditions and feasibility are discussed. A versatile foundation appeared to be similar to a traditional one. Differences and challenges arise in case of changeable foundations. For example, the standards of dimensions and connections must be determined. Despite the disassembly, the strength, stiffness and stability must be maintained. Also, the reusability of foundation piles can be improved. More generally, finding a balance between uniformity and freedom of form is both important and challenging. Circular foundations require precise documentation of characteristics and availability. Altogether, the traditional foundation can be more circular with some achievable changes. Regardless of the building, adding a certain degree of circular value to the foundation is always possible but requires willingness and innovative thinking. This cultural change is not simple, but it will be beneficial.