The conception of a Generative Model (GM) is an important step when applying optimization methods in architectural design. The variant spectrum generable with a GM determines if an optimal solution for the different demands placed on a design can be found. Using the example of optimizing façades (more specifically window-layouts), it is shown that GM that exclusively vary either metric or topological properties of the geometry are not sufficient, because they only cover a highly restricted solution-space. To keep the solution space as large as possible, it is argued, that it is necessary to vary both topological and metric properties. The combination of both properties is called topo-metric properties. A GM for the generation of facade variants is presented, that is able to systematically vary these topo-metric properties. The effectiveness of the developed GM compared to conventional GMs is demonstrated using a simple test scenario.