Preliminary structural design is used for estimating and comparing design alternatives. It is mainly a manual procedure based on experience and documented in the form of design tables or charts. In an effort to automate this procedure for steel composite office buildings, the Sustainable Office Designer (SOD) has been developed as a SketchUp plug-in. It can generate optimized preliminary structural designs for given rectangular boundary shapes and employs a rapid approximate calculation instead of an expensive structural analysis. Steel columns are used and verified according to DIN EN 1993-1-1, while beams are designed as composite members and verified to DIN EN 1994-1-1. Pre-calculated design tables are used for slab systems. Optimization results are obtained and compared for different objectives, e.g. life cycle assessments (LCA) and costs. 2 Sustainable Office Designer (SOD)