This study aims to investigate and evaluate design principles that are important or central for developing a mobile application for fieldworkers. By developing a design proposal for a case company, the design principles can be evaluated and reflected on. The theoretical framework, used in the analysis, includes definition of mobile units, design principles for information system and challenges for developers of mobile units. Qualitative data has been collected during the study by interviews and prototype testing at the chosen company. Two interviews were completed with field workers, each interview took about one hour. A development of low and high detailed prototypes were also done, to enhance communication and visualization during user testing. Service experience blueprint was applied for the concept to visualize the customer journey together with the mobile application. The prototype testing included three field workers and took about one hour. To complement the interviews and prototype testing the author(developer) gathered information about the context the field workers are in during the study. The information was in the form of photographs taken of the context and the field workers field notes describing different variables that affected the work. Action design research has been used as a strategy that has enabled design principles to be explored, evaluated, tested and finally presented in a proposed design solution. Finally, the result presented important design principles and specifically, highlights new design principles that are central for developing a mobile office for field workers. The result of the study shows that the field worker want to have informative feedback when the context change and has an effect on the documentation process. The proposed application can provide the right information that the field worker needs by gathering information from small devices called Internet of Things (IoT). The information can then be used to support the user in their customer journey. The secondary data also shows that the field worker are in a noisy context. In that perspective, the developer needs to be restricted to use sound as informative feedback. The study also provides new design principles. The fieldworkers prefer to make interact with a digital pencil when documenting information during a project. The context is also changeable in for example light. To make it easier for the field worker (the user), the system should make it possible to change color and contrast of the layout. Also, due to perspective, the navigation and the structure of the layout should make it possible for the field worker to choose alternatives ways beside the standardized menus.