This paper proposes a way how a semantic aesthetic property from architectural theories in the Western world can be integrated into a computer-aided architectural design (CAAD) system. It starts from the premise that computer-aided design tools are mostly aimed at serving as drawing tools which are used only after a design formal solution has already been established by the architect. To support an early design solution in a computer-based environment, a design checking tool was developed and tested in a real building project. This tool gives various design alternatives from the early design phase to the final stage of design details, according to musical harmony. Finally, this paper shows that an aesthetic property from architectural theories can be calculated with the aid of a computerbased design tool, and the used tool played its role as a design assistant in supporting the architectural design