This 19th century building, an old building of the Societat Cotonera de Barcelona, has been used in recent years as a rather dilapidated neighbourhood centre. It is being rehabilitated by preserving 3 façades that are insulated from the outside with wood fiber, keeping all the thermal inertia of the existing stone façade. The slab on the ground floor is also preserved, which shows the metal structure and the ceramic vaults at the bottom. The rest is being rebuilt, expanded to the east, and one more floor is added. The whole new structure is prefabricated in wood. It is seen inside and protected on the outside by a ventilated perforated corrugated sheet metal façade. The central atrium, which was walled up, has been restored and is now returning to natural light, making it easier to see and understand the interior of the building. The light from the atrium enters laterally through an emptying of the south façade on the top floor to allow the sun to enter laterally in winter and not to enter vertically in summer as it did before to prevent summer overheating and favor winter solar capture. This volumetric emptying creates a useful outdoor space as an outdoor room or workshop where aspects related to the atmosphere and the universe can be worked on. The building, which almost doubles the original usable area, houses a function room, workshops, rooms for neighbourhood organisations and other services. Maximum energy savings are sought to the point of being a zero-balance building, as it will produce as much or more energy than it will consume throughout the year, thanks to the photovoltaic solar capture of the roof. Good perimeter insulation, between 12 and 24 cm, with natural materials and good active and passive solar capture with mobile sunscreens for the summer, are the key to autonomous operation without external energy. The work has a positive CO2 balance because more than 200m3 of wood have been used, basically in the prefabricated structure, which stores more CO2 inside than has been generated throughout the work and its materials. The area around the building is connected to the existing square by demolishing the fences that bordered it. The entrance square follows the language of the existing rear square, respects the existing trees, and is paved in the logical paths of pedestrians. The colour of the pavement follows the trace of the old control lair that delimited the industrial area. A continuous open public space has now been built around a 19th-century building that speaks of the 21st century.