As seismic retrofit for Aichi prefectural office building, base isolation retrofit method was recognized by Aichi prefectural assembly and we started further study for basic design. We had to budget the seismic retrofit cost for coming year and we investigated important matters for cost such as building foundation level, necessary volumes of soil improving and so on. And we had to decide seismic criterion of retrofit building. We had to study about tender method in addition. This report is summary of basic design.