In many cases of small scale software development, downstream activities are actually carried out before upstream activities have completely finished in order to reduce the development time and meet the deadline. However, overlapping activities also increase the total work effort because it demands more communications between activities. It is very important to be able to estimate the tradeoff between development time and total work effort since constraints may be placed on them. The paper proposes a simulation model to describe the overlapping development processes and the effects between overlapped activities. Based on this model, a software process simulator was developed. This simulator is applicable for supporting project planning (scheduling and staffing) with constraints on cycle time and total work effort.