Shrinking population numbers are a challenge for planning future investments into urban built structures. In the project “Energy-efficient Delitzsch” funded by the German Ministry for Education and Research, we investigate the effects of population shrinkage on investments in energy-efficient heating systems and insulation of residential buildings. Therefore, we will build an agent-based model on residential mobility within a shrinking city to allow for simulating future patterns of population distribution. The agent-based model will use empirical data on residential mobility that is being surveyed especially for this project. This work is embedded in a dialogue with policy makers and residential building cooperatives about future investments into energy-efficient infrastructure in the city of Delitzsch in Saxony, Germany.