Since historic buildings constitute 25% of the European built environment they have a role to play in delivering CO2 emissions reduction targets along with the rest of the domestic stock. However, historic buildings have significant cultural value and were built with technologies and materials that promote fabric breathability. This demands solutions that will deliver enduring and radical energy efficiency savings and emissions reduction, which while maintaining their heritage value are also capable of district wide replication. Before embarking on wide scale retrofit adaptations, affordable and accurate procedures to assess the potential for such measures to reduce CO2 emissions are of primary importance. Some measures will have an impact on both fabric and aesthetics. It is therefore necessary to ensure that the reductions in CO2 emissions from a set of proposed alterations are significantly higher than any actual or perceived reduction in loss of built heritage. This paper demonstrates the use of the Passive House Planning Package (PHPP) modelling tool to assess the potential for retrofit adaptation measures in three terrace dwellings in Bath, England. It compares modelled against delivered energy use and then models energy and emission reduction following the introduction of a suite of retrofit adaptations. Results indicate that PHPP can assess total electrical energy consumption but requires the use of a reduction factor to reflect accurately intermittent occupancy/heating patterns. The modelled results suggest retrofit adaptations in historic buildings could deliver energy savings and CO2 emissions savings between 55% and 83%, but only when the thermal fabric is significantly improved and the use of PV is included. PHPP provided assessments of the benefits of retrofit adaptations in historic buildings that can facilitate decision making on retrofit methodology in historic buildings that affect fabric and/or aesthetics.