The subject of this work is the restoration of the timber floors of a 17th century building located in the city center of Milan. According to the Italian Code (Dlgs 42/04), this building is within the Architectural Heritage list. The building was deeply damaged and in order to be used, a restoration intervention was need. The intervention involved the building as a whole: timber roof structures, masonry walls and timber floors. Attention is focused in this paper on the strengthening of the timber floors. The criteria adopted in the design are based on the Guidelines for Cultural Heritage Buildings and on the Chapter 8 of the Technical Italian Code Norme Tecniche sulle Costruzioni (NTC 2008) related to existing buildings. The building has a complex plan, resulting from the aggregation of buildings built in subsequent periods: since their state of preservation is different, different type of interventions is needed. On the basis of the conditions of the timber joists that constitute the floor structure, three types of intervention were identified. Floors in bad state of conservation or with increased loads due to new functions will be substituted adopting the same dimensions of beams and joists, same span and construction technology. The floors that have previously been strengthened with a concrete slab overlaid on the timber floor will be maintained. New floors made with steel beams and bricks placed over the timber floor will be removed and strengthened in order to keep the same level of the timber floor. Finally, original timber floor in good state of conservation but with insufficient cross section dimensions will be strengthened using similar timber beams and joists connected with the original elements through metal fasteners in order to obtain a whole cross section. The leading concept of the whole intervention is to preserve the original structure as much as possible or to consolidate it adopting the less invasive and the more compatible construction technique.