With the seismic provisions in the Loadings Standard, NZS 4203:1992 being replaced by NZS 1170.5:2004 (Structural Design Actions – Earthquake Actions), a number of changes have been made to seismic design in New Zealand. The most significant of these is that the level of detailing used in potential plastic regions (nominally ductile, limited ductile or ductile) be determined on the basis of the predicted magnitudes of deformation they are required to sustain in the ultimate limit state. Previously the level of detailing was determined principally on the basis of the structural ductility factor. However, it is shown that the structural ductility factor does not give a reliable guide to the deformation sustained in the individual plastic region. The paper outlines the background to the change in the way that the level of detailing is determined and it discusses how the predicted inelastic deformation demands can be found in the design of concrete structures