The historic building on Stadiou Street, Athens, Greece, hosted for years the “Akron Ilion” firm. The existing building was arranged in five floors and had a structural system consisting of masonry walls and steel beams that support the final floors. According to the architectural study of the new building, the perimetric masonry walls had to be retained and the internal of the building should be demolished and substituted by a structure consisting of structural steel columns and beams and composite floors. The retention of the perimetric masonry walls posed significant problems both in the design phase and in the development of the construction methodology. Alternative solutions were investigated, in which the walls were partially or fully connected to the new structure. After judging the advantages and disadvantages of each approach, it was finally decided to realize a full connection between the perimetric masonry walls and the new structure. To this end, the floors of the structure were monolithically connected to the masonry walls, which were strengthened by means of a thin layer of shotcrete. The design of the structure was based on an appropriate model that included all the structural elements. In order to approximate the complex seismic behavior of the building, nonlinear analysis was employed, that considered the actual strength and stiffness of the reinforced masonry walls. The erection was realized after a partial demolition of the existing internal masonry walls and the utilization of certain sub-frames of the new steel structures for the temporary bracing of the perimetric masonry walls. At the upper end of the masonry walls a strong concrete beam was arranged which was connected to horizontal steel bracing elements, to form a week temporary horizontal diaphragm. This seismic design and verification of this temporary structure, consisting of the perimetric masonry walls and certain steel members, was done following the methodology included in ΕΝ1998-2:2005. The fire design of the new steel frames was done following the methodology included in ΕΝ1993-1-2.