Abstract: We present a novel procedural framework for interactively modeling building facades. Common procedural approaches, such as shape grammars, assume that building facades are organized in a tree structure, while in practice this is often not the case. Consequently, the complexity of their layout description becomes unmanageable for interactive editing. In contrast, we obtain a facade by composing multiple overlapping layers, where each layer contains a single rectilinear grid of facade elements described by two simple generator patterns. This way, the design process becomes more intuitive and the editing effort for complex layouts is significantly reduced. To achieve this, we present a method for the automated merging of different layers in the form of a mixed discrete and continuous optimization problem. Finally, we provide several modeling examples and a comparison to shape grammars in order to highlight the advantages of our method when designing realistic building facades.