In order to plan the interior of a room, various programs for computers, smart phones or head-mounted displays are available. The transfer to the real environment is a difficult task. Therefore an augmented reality approach is developed to illustrate the planning in the real room. If several people want to contribute their ideas, conventional systems require to work on one device together. The aim of this master thesis is to design and develop a collaborative spatial planning application in augmented reality. The application is developed in Unity with ARCore and C#.