This research paper presents the development and application of Urban Decarb, a parametric tool based on Life Cycle Assessment (LCA) designed to integrate carbon knowledge into the early stages of urban development to guide low emission design. By modelling key components of urban fabric and utilizing the visual programming environment of Grasshopper, Urban Decarb provides a dynamic platform for comparing the carbon footprint of various urban design scenarios. Case studies from Fælledby and Aarhus Sydhavn (DK) illustrate the tool's utility, showing significant reductions in Global Warming Potential (GWP) through material innovation, reuse of existing infrastructure, and holistic design strategies. A novel approach introduced in this study is the use of carbon goggles, a conceptual visualization method aiding in identifying high-carbon elements within existing urban infrastructure, thus informing sustainable redevelopment strategies. These insights reflect the importance of incorporating sustainability from the outset of urban planning to create low-carbon cities. The paper calls for integrating such tools into broader urban planning and policy-making processes, underscoring the necessity of multidisciplinary collaboration for the advancement of urban sustainability.