This paper introduces the concept of responsive design. It elaborates this concept as an approach to free form, adaptable, automated design applying physically based modelling techniques to the design process. Our approach attempts to bridge the gap between totally automated design and the free form brainstorming designers normally employ. We do this by automating the initial placement and sizing of design elements, with an interactive engine that appears alive and highly responsive. We present a method for applying these techniques to architectural space layout planning, and preliminary implementation details for a prototype system for developing rectangular, two-dimensional, single- story floor plans.