As software projects continue to grow in scale, being able to maximize the work that developers can carry out in parallel as a set of concurrent development tasks, without incurring excessive coordination overhead, becomes increasingly important. Prevailing design models, however, are not explicitly conceived to suggest how development tasks on the software modules they describe can be effectively parallelized. In this paper, we present a design rule hierarchy based on the assumption relations among design decisions. Software modules located within the same layer of the hierarchy suggest independent, hence parallelizable, tasks. Dependencies between layers or within a module suggest the need for coordination during concurrent work. We evaluate our approach by investigating the source code and mailing list of Apache Ant. We observe that technical communication between developers working on different modules within the same hierarchy layer, as predicted, is significantly less than communication between developers working across layers.