Building decarbonization is a major challenge for cities. Deciding what, when, and how to retrofit a building is difficult, given the complex interaction between energy costs and investment requirements. Several tools have been developed in the last years to help public and private stakeholders with these decisions, but none cover aspects the authors think are fundamental. For this reason, an urban building retrofit tool was developed and is presented in this article. This new tool is based on a bottom-up approach, with all buildings simulated individually, considering aspects such as shading or adjacencies. As a second step, three scenarios with different levels of ambition have been implemented in the tool, and the energy demand and emissions resulting from these scenarios have been calculated. As a third step, the retrofitting scenarios' initial investment and operational costs have been implemented via a detailed Life Cycle Costs (LCC) approach. The authors describe the robust and scalable structure that has been developed and the application of this structure to calculate the LCCs of different retrofitting scenarios in Montréal.