This paper proposes a theoretical framework for a computer aided garden design tool. The presented system supports the conceptual stage of the design process during which gardens in a given style should be created. First, a conceptualization of the design domain is defined. On this basis a visual language composed of design drawings and the internal graph-based structures representing the drawings are specified. Operations on graph structures, which reflect design actions performed on design drawings, are proposed. Sets of requirements which determine object styles are defined in terms of logic formulas which are mapped into graph-based constraints. Styles of the generated solutions are accessed by the system using the graph-based reasoning mechanism, where the satisfaction of constraints by internal representations of design drawings is checked. The approach is illustrated on examples of designing gardens in different styles.