This paper deals with evaluating design on the basis of their internal structures in the form of graphs. A set containing graphs representing solutions of similar design tasks is used to search for frequently occurring subgraphs. On the basis of the results of the search the quality of new solutions is evaluated. Moreover the common subgraphs found are considered to be design patterns characterizing a given design task solutions. The paper presents the generic concept of such an approach as well as illustrates it by the small example of floor layout design