The construction sector has significant impacts on the environment due to the consumption of resources and the production of waste. The implementation of circular economy strategies can improve the environmental performance of the sector, optimising the current production and consumption patterns, slowing and closing material loops, and using waste as a source of raw materials. Biowaste represents a key waste flow at the European scale. However, research on its application in the construction sector is still limited and product-oriented, with little insights into the processes of valorisation undertaken at the company’s level. This study presents eleven case studies of Belgian small and medium-sized enterprises involved in biowaste valorisation in the construction sector in order to tackle this research gap in the Belgian context. Semi-structured interviews were conducted to identify the enterprise’s business profile and its current marketing practices, as well as to analyse opportunities and barriers for market expansion and highlight current research interests. Results show that the overall picture is extremely heterogeneous in terms of sourcing, production methods, and products, while the barriers and success factors that have been identified are recurrent. This study contributes to the circular economy research in the construction sector by providing insights into innovative waste-based materials and business models.