This work presents a detailed case study for the optimization of the expansion of a district energy system evaluating the investment decision timing, type of capacity expansion, and fine-scale operational modes. The study develops an optimization framework to find the investment schedule over 30 years with options of investing in traditional heating sources (boilers) or a next-generation combined heat and power (CHP) plant that provides heat and electricity. In district energy systems, the selected capacity and type of system is dependent on demand-side requirements, energy prices, and environmental costs. This work formulates capacity planning over a time horizon as a dynamic optimal control problem considering both operational modes and capital investment decisions. The initial plant is modified by the dynamic optimization throughout the 30 years to maximize profitability. The combined optimal controller and capital investment planner solves a large scale mixed integer nonlinear programming problem to provide the timing and size of the capacity investment (30 year outlook) and also guidance on the mode of operation (1 h time intervals). The optimizer meets optimal economic, environmental, and regulatory constraints with the suggested design and operational guidance with daily cyclical load following of heat and electricity demand. (C) 2016 Elsevier Ltd. All rights reserved.