A customizable routing system allows designers to create custom connection layouts that can be stored, turned into templates, reused, and further customized. The system describes designer-input custom connection layouts in terms of “structural directives” that specify its patterns and properties instead of using precise dimensions. Structural directives may describe particular connection patterns between structural components (e.g., backbone or fishbone), the placement, width, direction or layer of specific structural components, and properties of structural components relative to other components. These structural directives are implemented generally during routing, such as through design constraints, which allows the router to locally optimize the design (e.g., for cost or wire length) while considering the structural intentions of the designer. The system can also learn and replicate customization patterns based on existing layout templates by comparing connectivity information to that of existing layout templates and applying applicable structural directives.