The design of onsite systems is often very prescriptive, codes dictate many aspects of the design. However, the design of the infiltrative surface is one variable that designers are expected to be able to make decisions. Soils depth, type, siting location, wastewater characteristics and many other variables need to be well understood to provide a system that will have good longevity and more importantly protect public health. This presentation will review the boundary conditions that determine best practices for infiltrative surface design.