Designing good, low-energy buildings is a complex, multi-objective design problem. There are many things that can be changed, and many different performance criteria that we are interested in. This complexity can result in many thousands or millions of possible designs. Testing each design is time-consuming meaning that designers tend instead to use their experience, intuition and rules of thumb to guide the search for the best solution. My PhD and Masters theses focused on using evolutionary algorithms to search for optimal low-energy building designs, and I have since successfully applied these methods to a number of consultancy projects. This approach can be integrated into both new-build (including Passivhaus) and retrofit design consultancy, and can include multiple objectives such as cost, energy performance, comfort and summer overheating risk.