Designing an outpost for a long-duration scientific expedition to an extreme environment such as Mars requires the basic qualities of functionality, comfort, and security. In this paper we assess: the key environmental conditions that govern the functionality of a Martian habitat; what it takes to design and build a comfortable home for a crew of four for 500 days with limited communication with Earth; and most critically, how to ensure mission success, safety, and robustness through increased redundancy. These factors are collected here into a conceptual design proposal, along with details of the deployment and multi-robot additive regolith construction system. This work is part of ongoing research at F+P specialist modelling group on robotics, large-scale additive construction, and architecture for extreme environments.