To mitigate the impact of the climate, there is an advantage in incorporating climate calculations for design alternatives early in the design process. However, there is a need for tools dedicated to providing climate feedback on design sketches in the early design stages. A main actor in this stage is the architect. It is therefore beneficial if the toolchain already used by architects can be adapted to provide the necessary decision support for climate optimized design. For this purpose, a software tool called LCAnt version 0.2.2 was developed. This tool assists in determining amounts of materials from volume sketches in Rhino using a novel method for estimating the load-bearing structure and connecting it to an existing life cycle assessment database through Grasshopper. A real-life case study was used to demonstrate and evaluate how this type of tool can be applied in early design stages. This study was expanded by exploring strategies for managing uncertainties naturally present in these early design stages. This work demonstrates how to incorporate uncertainty evaluation into the early building design phase to obtain higher-quality decision support for low-climate-impact buildings. The uncertainty evaluation will at the same time identify in which stage important decisions regarding layout and material choices should be made to minimize the climate impacts of the finished building. Combining this with the studied tool LCAnt, which adapts to the workflow of professional building design, will facilitate implementation.