We are dealing with a large building stock, an increasing demand for residential space, and the awareness of the carbon footprint of old and new constructions. Breathing Building aims to provide a simple and systematic alternative to address building reuse through several strategies in two scales –the building and construction modules– that maximize the usable and livable area on every floor, allowing the building and units to breathe, regenerate, and adapt permanently over time. The project is a modular wood-based system for office building adaptation, offering scalable solutions that could be used and fitted in different buildings, using sustainable materials, preserving almost the entire building structure, and recycling the demolished. The system was tested on the 550 Seventh Avenue Building, a typical 24-story structure in Midtown Manhattan. The main strategies are cutting every slab to build the active core and new shaft, the void of intercalated double-height stories, and the modular raised floor and telescopic wall panels. Thus, flexibility to generate and change spaces, active community areas, inhabitant interaction, natural lighting and ventilation, maintenance independence, and climate control performance for the building is achieved. Prototypes and 3D models were built to test the assembling and construction details of the system.