A purpose of this study is to investigate the case of portable Building with spatial extension and to clarify its geometrical features and applications. A portable building in this paper follows the definition of Robert Kronengburg. Portable building indicates the space which can be carried and towed without disassemble into parts. As a result of the analysis, it could be classified into four groups. Group1 and Group 2 are both under 50 m2. And only Group1 is mass produced for travel trailer used by individual consumer. Group 2 has variety of expansion system but has not become general. Group 3 show the limits of area expansion using single trailer. And the limitation is around 70 m2. Group 4 is over 150 m2 and using two trailers to expand a space. It is considered that the mobile space is effective as a means to enhance the ability to cope with future infectious diseases and severe natural disasters. The knowledge in this paper will be basic information about usage of portable building during normal and emergency. Flexibility of expandable portable building can rise a resilience of society against pandemic and natural disaster.