Comprehensive experimental and analytical studies have been conducted to understand the behaviour of frame buildings constructed before the introduction of modern design codes. This usually has been done assuming a fixed-base structure while ignoring the flexibility of soil. The interaction between the super-structure and sub-structure (SSI) is investigated by modelling the soil as simple as possible to capture the overall response of the system. As new analytical hysteresis rules and more advanced tools of analysis have been developed in recent years, and as part of a more comprehensive investigation on the response of older structures before and after retrofit, focus will be herein given on the response of SDOF systems representing a broad range of existing, newly designed and, retrofitted structures, while allowing for flexibility of the soil-foundation system. The results of this study suggest that the compliance of simply modelled soil for typical building structures have in average beneficial effects in terms of structural demand especially in the case where as a consequence of implementing a retrofit strategy, the stiffness of the structure might increase. On the other hand, the governing component of these effects, i.e. rocking of foundation, can result on average in higher absolute displacement of floors.