This paper describes a new open source software library for the computation of thermodynamic and transport properties in non-equilibrium gas flows and species production rates derived from a rigorous kinetic theory formulation. The code is called KAPPA, a Kinetic Approach to Physical Processes in Atmospheres. Several levels of non-equilibrium flow description are currently available: The detailed state-to-state (STS) approach, multi-temperature and one-temperature models. The development of KAPPA is described which has been written in C++ and designed in order to be highly modular, easily maintainable and extendable. Moreover, current lines of research and future works are presented such as the interface of KAPPA with existing CFD codes in order to enhance code reusability and readability.