To stick to climate change targets and reduce global carbon emissions, the building sector needs to adopt a circular economy. In the steel sector, reuse or elements can reduce carbon emissions which are created in today's steel scrap recycling practices. This thesis contributes to closing the knowledge gap on design, costs, and environmental impact score for working with reclaimed steel elements. The tool Steel-IT is created to research the business-case and environmental impact score of low-rise office buildings. Steel-IT uses a donor element database and new office design, which are specified by the user, and generates new and reclaimed steel design alternatives. Using a weight and activity-based costing method, the costs of these design are estimated. Life-cycle analysis is used to quantify the impact. By generating results and analysing the effect of the input in the tool Steel-IT, the following conclusions can be made regarding the business-case and impact score of reclaimed steel in low rise office buildings. A case study using Steel-IT shows that a 25% impact reduction against minimal cost increase is possible. For this, the match between design and donor elements needs to be good. This match can be searched for using Steel-IT. In terms of design, connection detailing is the most critical factor in the success of a donor steel skeleton. Donor steel elements should be sourced from an existing building to limit storage costs and should not have toxic conservation systems applied to them. Both induce extra costs and make the business case of donor steel less competitive with new steel. In the future, currently suggested emission taxes do not affect the business-case significantly as these taxes are low compared to the total building costs. The most helpful action to increase the business-case is to apply donor steel more often. If the experience with donor steel and its availability grows, reuse will become easier. Steel-IT can contribute to this as it contributes to closing the knowledge gap on costs and impact and can help to match demand and supply.