In order to decrease time for inputting data and to check and correct the data in the design process with CAD system, we investigate some relationships among the data of architectural form. In this paper, three fundamental equations are obtained. The first one is an equation expressing a relation on coordinates and dimensions of building parts. By this equation and given data of coordinates, data of dimensions can be calculated automatically. The second equation is one for calculation of data of coordinates from dimensions. The third equation is expressing a topological relation between information on connections of building members and one on edgepoints of members. Using these equations, it is shown that number of input data is extremely decreasing in a case study on design alternation process.