The client intended to create a veterinary hospital within the empty space of an industrial warehouse. As it was a rented space and the owners didn't want to modify the existing site, a light construction system, easy to build and to remove was required. On the other hand, the legal issues concerning a veterinary hospital demanded total isolation from the existing building, washable and fire-proof surfaces, independent ventilation, and direct access to the street. Therefore, our thought was to create a box inside the box, emphasizing the warehouse's wide interior space, controlled natural lighting and surprising thermic inertia. Besides, the glass and iron ceiling structure gets its own architectural expression, cleansed and participative on the interior space quality. The box is made of steel and painted wood, with punctual skylights. There is a patio that decompresses the interior spaces, giving them more light as it triggers new interior/exterior relationship. The box is entirely black, contrasting with its white interior and the warehouse's original painted walls, integrating a global design concept, that results on a clear approach to hospital environment, integrating comfort and safety.