This paper presents a generative approach for the conceptual design of structures, which aims at exploring alternative bar networks in static equilibrium by means of a grammar rule. Operating on incomplete networks of bars in interim static equilibrium, the incremental rule application imposes the iterative introduction of: a new node, some bar elements and a few interim forces, based on the desired type of transformation. The node introduction is only possible within a feasible domain that allows both the rule application and the bar elements’ constructability. In this paper, the grammar rule not only considers static equilibrium, but also designer-defined bar length constraints. These supplementary constraints are transformed into geometric domains and respectively inform the feasible domain. The length constraints neither shrink the exploratory power of rule-based design nor the size of the design space, but rather allow for structures materialization at a later stage. The generation of planar networks is used as a case study of the methodology.