Multi-touch user interfaces (MTUIs) of mobile devices can represent a valuable tool for enhancing co-design practice. In particular, mobile augmented reality (AR) technology that enhanced with MTUIs can offer new possibilities to designers to work and collaborate. This paper describes a series of user studies that took place in the Department of Architecture, at the Istanbul Technical University in January 2016. The users tested the developed AR application. The task of the participants was to design mass volumes of buildings in the given contexts. In the first study, the designer worked solo using analogue tools and two different MTUIs of the AR applications. In the second study, the designers collaborated using analogue tools and an enhanced mobile augmented reality environment. The goal of the study is to understand the affordances of the interfaces and the changes onto designers' behaviour when they are using the mobile AR applications. The particular focus of the paper is on the characterization of the co-design cognition and interaction behaviour of the designers. The collected data from the empirical studies is analysed with the protocol analysis method using a coding scheme. The results show that the studied AR interfaces afford different modelling actions and design behaviour. The design context was broad conceptual and high-level; the intensity of the actions was also different in the AR sessions. We consider that this knowledge would be informative for the development of the innovative augmented reality applications for supporting the design activity and be guidance for the further developments.