The creditability of using CAAD depends on the rigour of its methodology, the logic of its deduction and most importantly the feasibility of its results for practical use. This paper examines two lighting simulation software as the basis for providing a wider critic on the research of CAAD in the practice of architecture. The paper argues that the “contextual” and appropriate use of a simple tool or method should be the thesis of CAAD research. Using an example, the paper then logically work out an example of how that could be done, and the basis of its contextual logic. The example illustrated here concludes the validity of the software and its implication for legal use. Furthermore, the paper provides a critic of CAAD for regulatory and legal acceptance. The experience in Hong Kong is illustrated.