GraCAD -- graph-based tool for conceptual design. This paper proposes a new methodology for computer-aided design of layouts of buildings. The presented methodology addresses the very early phase of designing that is called Conceptual Design. In this methodology the architect does not pay attention to layout details, but operates on functional requirements, constraints, and high-level architectural concepts such as functional areas and rooms. The GraCAD system realizes this methodology by adding a high-level design tool to the well-known commercial program ArchiCAD. This design tool has been realized using graph-based knowledge representation techniques and graph grammars as offered by the system PROGRES. Preliminary results show that the developed ArchiCAD add-on and the formalism of graph grammars seems to be useful in aiding architectural design.