By 2030, 1,700 churches will become vacant, adding to the 1,530 already repurposed. Repurposing these spaces is time-consuming and costly, often leading to demolition when no potential is identified. This research explores how modular wooden building systems with adaptable, remountable elements can create customizable, multistory spaces in vacant churches. Moreover, it addresses the key questions: What are the criteria for a modular building system that could be used as an infill within vacant churches? Which of the existing modular building systems could be a potential solution as an infill to fit within vacant churches? How modular is the most potential modular building system as an infill within vacant churches? What are the technical requirements of the system concerning the various functions it is designed to accommodate? The study concludes that developing wooden adaptable infills is feasible if factors such as adaptability, interchangeability, efficient placement, and material efficiency are prioritized.