Existing simulation tools are mainly used for appraising environmental properties of architectural and urban constructions. In most cases, these tools do not suit design practice as they work on completed schemes. A designer who wants to emphasize an environmental point of view needs a new range of CAD tools that work in an inverse way: from the environmental properties to the built forms. Our main issue for this paper is to present an inverse approach application in the field of sunlighting simulation and design. We have developed an experimental CAD tool that models objects from declarative sunlighting properties. Our system works on proposals such as ‘this shape must be sunlit (or sunless) for the end of afternoon in winter’, and it suggests solutions — shadings and openings — that check on these proposals. This methodology opens new research avenues toward a real aid for environmental design.