Long-term basement heave is a pertinent problem in the construction of underground spaces in over-consolidated clay strata, notably the clays of southeastern England. When a basement is constructed, the permanent removal of soil above formation level inevitably causes upward movement of the remaining soil, or heave pressures on the base slab. In over-consolidated clay strata such as London Clay, this process of heave continues over many years after the basements structural completion. The designer must predict these future heave pressures and movements when designing the basement structure. However, there has been much conservatism in the methods of design due to the scarcity of site monitoring data to calibrate the methods of heave prediction. There is a need for further physical data to improve the methods of design. This paper presents a research project that seeks to fill this research gap through the technique of geotechnical centrifuge modelling. The model simulates the construction and long-term heave of a 15 m deep basement underlain by stiff clay. The model basement is fitted with extensive instrumentation and this is the first research project to provide simultaneous measurements of the vertical movement of the base slab and the distribution of slab-soil contact pressure in a centrifuge model of basement heave. The experimental results are validated against finite element simulations of the same prototypes. The results show that the prediction of high heave pressures is a self-fulfilling prophecy: the assumption of high heave pressures by the engineer leads to the specification of strong structures to sustain the load, and the stiffness of these structures in turn restrain the soil, generating high heave pressures. This paper advocates an alternative design approach so that leaner basement structures can be specified, and thus urban underground spaces can be provided more economically.