We investigate user-guided evolution for the development of virtual 3D building structures for the prototype (commercial) game Subversion, which is being developed by Introversion Software Ltd. Buildings are described in a custom plain-text markup language that can be parsed by Subversion’s procedural generation engine, which renders the 3D models on-screen. The building descriptions are amenable to random generation, crossover and mutation, which enabled us to implement and test a user-driven evolutionary approach to building generation. We performed some fundamental experimentation with ten participants to determine how visually similar child buildings are to their parents, when generated in differing ways. We hope to demonstrate the potential of user-guided evolution for content generation in games in general, as such tools require very little training, time or effort to be employed effectively.