The presence of fast and slow modes in vehicle suspension systems based on a half car model, is utilized in the design of active suspension control using singular perturbation theory. This strategy is based on the slow-fast control design. The suspension system performance is optimised with respect to ride comfort, road holding and suspension rattle space as expressed by the mean-square-values of body acceleration (including effects of heave and pitch), tire deflections and front and rear suspension travels. The method of design in this study is based on LQG feedback control combined with singular perturbation theory, and at the end, a composite LQG controller has been proposed. Numerical simulations in the time domain evaluate the performance of the active suspension system. In spite of the simplified structure of the composite model, simulation results indicate that its performance is comparable to that of the full-state feedback design.