Generative design tools can accelerate the optioneering process by allowing designers to quickly generate large numbers of design variants, thereby enabling a wider and more thorough exploration to be conducted. This paper focuses on procedures for generating inner city street networks and city block massing studies for sites within existing urban areas. A novel procedure is proposed that is capable of subdividing complex non-orthogonal sites into similarly sized well-formed plots and subsequently further subdividing these plots into sizes appropriate for selected city block typologies. The application of the procedure is demonstrated on a site in Singapore.