This research paper proposes a system for interior design prototyping based on portable-type projection-based AR (Augmented Reality). The DIY (do it yourself) interior designing process takes much time, monetary resources and labor. To overcome such difficulties, many research studies in the have been undertaken in the field of AR. Among those studies, projection-based AR technology provides users virtual information that is very real by projecting new digital information onto things and spaces. Most of those studies employing projection-based AR technology have been carried out in predefined spaces. In this paper, a projection-based AR system that can be used to design interiors of non-predefined spaces was proposed. This DesignAR system can effectively design interiors, even in the spaces not predefined by a 3D map of the space. Also, by connecting with mobile devices, the system raises the user's degree of freedom due to the interface usability, mobility and overall convenience. In order to evaluate the effectiveness of the proposed system, we set up a scenario for interior design prototyping. A usability evaluation was conducted by questionnaire after users performed tasks based on a preset scenario.