Parametric NOK is a generative design tool for automatic generation of massing design based on the new greek building regulations (NOK) for multi-family residential buildings. The generation process is initiated by providing basic input about the site and location. The tool then creates multiple design variants for building massing which are constrained to follow the Greek building regulations. The user can combine the constraints in a preferred order, and explore different massing depending on the project goals. This approach enables the quick search in the vast solution space and, at the same time, allows the user to influence the generation process and guide it into the desired direction that complies with the greek building regulations, thus allowing the designer to integrate his experiences and insights in legal terms. The developed tool has been successfully applied to a case study in the city of Ioannina for the early design stage of multi storey residential building which has now obtained building permit is construction is design set to achieve the maximum allowed build area .