This paper describes a domain-specific software development method for the creation of building simulators. The method is based on object-oriented modelling, design patterns and code generation principles. The goal is to provide customizable building simulators that exactly simulate those physical effects an application demands. The numerical accuracy and different algorithms to be used can be tailored to the application's needs. By using object models and preconfigured design patterns, a well-structured simulator model can be created. From this model, the complete product code of a simulator is generated. The patterns help to develop a complete and correct model. Each pattern describes a certain functionality and knows how to generate code to implement this functionality.