The headquarters of the pump manufacturer Hidrostal AG in Neunkirch is home to the company’s research and development centre, the sales department and a factory, including the in-house foundry for pump castings. Hidrostal’s growth in recent years led to a need for more space for offices and R&D. Considering where to create the extra capacity, the limited amount of free space on the site meant that adding a storey to the existing 1970s office building was the preferred option. An upward extension of this type imposes a load on the existing building that was not originally planned for, and this makes the weight of the materials a key factor in the choice of construction method. In this case, the new storey is a steel-framed structure designed to transmit the load into strong points of the existing concrete building. The existing flat roof was left intact, which meant that the construction work could be done without interrupting use of the building, and independent of the weather. The flat roof of the extension is formed by Kielsteg elements. Kielsteg was chosen because it would span the required width with the lowest weight, forming a fully adaptable, high-quality office space.