The volume of existing buildings is much more than new buildings in developed countries. Applying new technology, new material and new equipment to renovate and make the existing buildings greener is crucial for sustainable development. An approach including current energy statistics survey, expert diagnosis, energy and economic simulation using eQUEST model is carried out in this research for an existing office building in Taipei City. A sustainable renovation scheme with a payback period of 5.75 years is proposed in this research. Lessons learned from this research can be further developed into a decision support system to assist existing office building diagnosis and sustainable renovation in a subtropical area.