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China is the largest energy consumer in the world, and the biggest source of energy demand in China is the building sector, which accounts for 46.5% of the country's total energy consumption and 51.3% of carbon emissions. The area with the fastest growth in building energy demand in the country is the central southern hot summer - cold winter climate region. The Passivhaus standard (and its retrofit equivalent EnerPHit) focusses on achieving ultra-low operational energy consumption. However, examples of Passivhaus and EnerPHit buildings are rare for hot climates, and this particularly true for China. When improving the energy performance of an existing building, it is also important to consider the environmental impact of any retrofit building materials that are used as part of an EnerPHit retrofitting programme. Therefore, this study investigated the energy savings, carbon emission reductions and cost savings that might accrue from retrofitting, to the EnerPHit standard, a case study dwelling located in China's hot summer - cold winter climate region. External weather and indoor thermal conditions of the case building were monitored for 12 months, and these data were used to validate a digital model of the dwelling. The model was used to evaluate retrofitting to the EnerPHit standard, which gave energy savings of 90.1% for heating and 72.6% for cooling compared with the pre-retrofit condition. Next, different combinations of retrofitting measures were evaluated for four different build scenarios to find how the EnerPHit criteria could be met using fewer building materials. Lastly, the finalized retrofitting plans for the case building were evaluated from the carbon and cost points of view, through detailed life cycle carbon and cost assessments. The results showed that the carbon payback time of the proposed retrofitting plans was much faster than the cost payback time, and that at the end of the evaluated lifespan, savings of up to 83.4% of the carbon emissions and 18% of the costs could be achieved.