This project proposes a disassemblable and reusable modular timber system for small urban sites, where construction is constrained by limited access and space. Combining digital design, prefabrication, and modular construction, it addresses two urgent challenges: skilled labor shortages and embodied carbon reduction. The structure consists of box-shaped wall modules made from standard JIS lumber and CLT. Each prefabricated panel weighs less than 200 kg, allowing installation on confined sites. Assembly requires only screws, reducing dependence on specialized carpentry. Around 2,500 screw locations were coordinated in a 3D CAD model, while 3D-printed guides enabled precise construction. As a result, the structural frame was completed in only two days by scaffold workers rather than skilled timber craftsmen. Designed for circularity, the building can be disassembled, relocated, and reused. Compared with a reinforced concrete structure, it achieves a 42% reduction in upfront embodied carbon, with further reductions expected through future reuse. Unlike conventional mass timber projects based on large-scale standardization, this proposal responds to the realities of Japanese cities, where small vacant sites and locally distributed timber production are common. It demonstrates how adaptable timber buildings can regenerate the urban landscape through circular design, digital fabrication, and local production, redefining how buildings are built, reused, and renewed.