Design errors cause a big part of the defects occurring in building production and maintenance. Earlier research have shown that the most common cause for design errors is lack of knowledge. Product-model based CAD-systems are increasingly used in structural engineering practice and it is well known that these systems reduce the design errors, mostly through better visualization and collision checks. The design errors caused by lack of knowledge are not prevented using product-model based CAD-systems as we do today. This paper describes a case study of design errors where the structural engineer is involved. The aim of the study was to find new ways to prevent design errors using the product-model based technique. The study supports earlier findings that the use of product-model based CAD-systems have a great potential in preventing design errors, especially co-ordination errors. The study also shows that many of the design errors caused by lack of knowledge could be prevented in future projects if the design errors from earlier projects were systematically documented and made available to the structural engineer in the design process. This could prevent as many design errors as is prevented by using product-model based CAD-systems today. The study shows that about 90% of the design errors that could be prevented in this way were situated where two or more elements meet. It was also found in the study that the information needed about the design errors could be retrieved from construction deficiency reports. Based on these findings it is argued that design errors, caused by lack of knowledge, can be prevented in future projects by making the information from the construction deficiency reports available and retrievable for the structural engineer by indexing them using information about the elements meeting where the design error occurred.