Major changes have occurred over the last six decades in New Zealand design codes for seismic resistance of structures. This paper describes the changes in the required design strengths, stiffness levels and capacity design provisions with particular reference to buildings where the lateral force resistance is provided by reinforced concrete moment resisting frames. It is shown that simple comparisons of response spectra and limiting inter-storey drifts can give misleading conclusions regarding relative strength and stiffness requirements unless allowance is made for many other interacting factors. To illustrate this, minimum design requirements defined in codes (or standards) over the last six decades are compared with the corresponding 2009 design requirements for regular buildings in which the lateral force resistance is provided by moment resisting frames. The approach that is described can be applied to other forms of structure. The paper is intended to provide background information for engineers planning to assess the need for seismic retrofit of existing buildings and to show the different factors which should to be considered in assessing existing structures against current design criteria.