The automation of the design process is an important property of computer aided design systems. One of the methods which support it is using graph grammars as a tool for representing the design process. In this paper we consider the coordination of two independent sub-processes: interior and exterior design. The problem of their coordination results from the fact that both actions may interfere. Assuming that sub-processes are modelled by means of two independent graph grammars, our goal is to create a mechanism of their synchronization, which enables preserving an overall consistency of the design. The proposed mechanism is illustrated by the example of the interaction of external and internal design processes performed on the shared environment.