Urban building modeling tools are developing rapidly;these tools use emerging simulation workflows for specificurban environmental design tasks, such as assessingthe impacts of energy efficiency technologies at a districtscale. However, with the emergence of new environmentaldesign tasks, addressing all possible use casesand tasks is challenging and cannot be covered by a singletool. Urban-scale analysis at this level of complexityoften requires linking multiple emerging tools, ratherthan using a single tool, to adequately evaluate a variety ofpossible fields in urban environmental design. To achievethis, flexible platforms are needed to support multiple inputformats (e.g., geometric and non-geometric buildingproperties), enabling the mapping of such inputs to underlyingsimulation engines.This paper provides an overview of the open-source URBANoptSoftware Development Kit (SDK) for modelinghigh-performance buildings and energy systems at a districtscale. URBANopt’s flexible SDK is composed ofseveral modules that can be customized to integrate withother tools and generate new workflows to perform urbanenvironmental design tasks, such as capturing interactionsbetween individual buildings, district energy systems,distributed energy resources, and the electric distributiongrid.We describe the functionality of the core SDK modules inURBANopt (called Core Gem, GeoJSON Gem, and ScenarioGem) and discuss the flexibility of these modulesas a means of integration with a variety of tools. We alsodocument and demonstrate technical details of writing andcombining new modules to create customized workflows.Finally, we present a case study that uses the URBANoptSDK to model a hypothetical mixed-use urban project andsimulate various scenarios to meet district energy performancegoals.