The construction industry is well known for generating a large amount of carbon emissions and this, together with huge costs of buildings during their life cycle, seriously affects its environmental and economic sustainability. That reducing the former tends to increase the latter is a highly significant problem exacerbating the situation. Finding a solution with the lowest carbon emission at a certain cost is an urgent problem to be solved. Research to date aimed at remedying the situation, however, has been largely focused on either treating the two issues independently or partially conjoined, with little progress in their integrated assessment. In response, this study presents a method for integrating the life cycle carbon emissions (LCCE) and life cycle costs (LCC) of buildings to assess their life cycle carbon emission intensity (CEI) based on BIM technology. Through a public building in China as a case study, the feasibility of the method is verified, the key stages of building carbon emissions are analyzed using CEI, and the conversion of high carbon emission materials to low carbon emission materials is explored. The proposed methodology and framework provide solutions and ideas for achieving optimal costs commensurate with low carbon emissions throughout the building life cycle, facilitating the assessment of carbon emissions at the decision-making and design stages, achieving the optimization of building carbon emissions and costs, and enhancing building life cycle sustainability.