To achieve carbon peaking and carbon neutrality targets of mainland China and Hong Kong, carbon reduction in the building sector is a critical task due to its high contribution to total carbon emissions. Decarbonisation approaches in both operation and construction phases should be considered. A building information modelling (BIM)-based integrated design process (IDP) is introduced to connect the design, construction and operation as an entirety from the initial stage of a project to enhance the green building and decarbonisation design. Taking a real case of a Design-Build-Operate project, O•PARK2, this study investigates the benefits and problems in a BIM-based IDP in regard to green building and its potential impact on carbon neutrality during construction. A BIM-based IDP enables decision-making to be based on comprehensive and optimal information, which is well proved by the O•PARK2 project with the BEAM Plus being upgraded from Gold to Platinum. The carbon reduction potential during the construction phase is as high as 45%, which can only be actualised if decarbonisation specialists are involved from the planning phase. Although the BIM-based IDP possesses many unique advantages, the management of its long feedback loop and the complicated procedures should be optimised in the future.