Harnessing forest-derived residual biomass as a substrate for mycelial growth, combined with precision engineering of hyphal length, opens a radical frontier in biofabricated materials. By actively tuning hyphal elongation and branching patterns, we achieve structural control at the microscale, enabling composites with optimized tensile strength, flexibility, and load distribution. This deliberate manipulation of hyphal architecture transforms mycelia from passive growth forms into programmable material frameworks, unlocking performance profiles that rival traditional construction materials while remaining lightweight, thermally insulating, and fully biodegradable. Integrating circular resource streams with fungal biotechnology and hyphal-length engineering redefines material design for architecture and infrastructure—delivering a scalable, carbon-negative alternative that merges ecological responsibility with structural innovation.