This paper presents the MUSE – MyceliUm SEat framework as a research-through-design approach for applying mycelium-based materials in real-world contexts. Addressing the gap between experimental biomaterial research and functional deployment, the project establishes a cross-scale methodology that moves from extreme performance testing to interior spatial integration. Phase I situates mycelium composites within a high-performance racing environment to accelerate material optimisation under strict structural and ergonomic constraints. Insights from systematic material testing are consolidated into a material matrix adaptable to diverse functional requirements. Phase II translates this knowledge into furniture-scale applications, employing varied fabrication strategies and reciprocal material-form development, presented in “The ‘Living’ Room,” an installation that functions as a long-term public testbed, enabling evaluation of performance, perception and ageing under real use conditions. The framework demonstrates how extreme scenarios, interdisciplinary collaboration and spatial deployment can advance living materials beyond demonstrators toward scalable biodesign practice.