There has been a continuous improvement of the thermal quality of buildings during the past decades: While in the 1970s annual energy consumptions of more than 100kWh/sqm were common, todayís state of the art buildings usually feature energy demands below 10kWh/sqm. Based on a comprehensive dataset of more than 5.000 energy certifications for buildings constructed from 1994 to 2011, this paper establishes a cutting edge generalized additive modelling framework in order to explain the temporal development and spatial heterogeneity of thermal quality in the state of Salzburg, Austria. For this purpose, we have explanatory variables such as energy costs, national and regional economic factors as well as locational attributes influencing energy consumption at our disposal. Thus, our approach allows us to investigate the impact and efficiency of past adjustments of the legal framework and subsidies for energy efficient house building. The results can be used to improve legislature for energy efficient building construction and according national and regional subsidy programs.