We present a generative tool based on a new simulation method for making sunlighting an actual formgiver in architectural design. Our system reverses the common simulation process. It works with intuitive sunlighting properties such as “this area must be sunless the afternon in summer”. Given a property, it computes a complex geometrical volume figuring the sunlighting phenomenon in time and space. This volume provides a visualization of the sunlighting constraint and it enables the designer to model the different solutions — shadings or openings — that exactly check on the given property. We illustrate the system with two demonstrative examples in architectural design and we introduce some new developments under consideration at the present time.