A 6-story RC building with base isolation using rubber bearings and viscous dampers has been recently built in Shanghai. Since the building is founded on soft soil, concern regarding base isolation suitability arose; even the Chinese design code does not recommend this solution for soft soil. To clarify this issue, nonlinear time-history analyses are carried out for a number of natural and artificial seismic inputs that represent the site seismicity, accounting for the soil conditions. The relevance of soil-structure interaction is discussed and some simulations are performed. Adequacy of base isolation is assessed both in the superstructure and the isolation layer. In the superstructure, appropriateness is judged in terms of reduction of interstory drift, absolute acceleration and shear force. In isolators, correctness is evaluated in terms of axial force, torsion angle and lateral displacement; prescriptions of Chinese and European regulations are considered. The major conclusion is that base isolation of ordinary mid-height RC buildings founded on soft soil can perform satisfactorily in medium seismicity regions.