Room simulation techniques require input data of the room model and the boundary conditions. In an iterative early-stage design process, the general room shape and the boundary conditions correspond to the perceptual parameter space such as reverberation time, strength, clarity, etc. The influence of specific settings in the algorithmic details such as the number of rays, the temporal resolution, the filter bandwidth will be demonstrated by calculating room impulse responses under the condition of variation of such settings. Furthermore, the statistics of the input variables of the general shape result in certain degrees of freedom for the absorption and scattering coefficients. It is discussed how a design space could be defined, which describes the probability for achieving the desired room acoustic performance as a function of the general room shape and boundary conditions.