The double-stepped design of lifting surfaces, planing in water, has been seen to benefit the calm-water performance by distributing forces over the washed area. The unsteady motion of such surfaces in waves, however, is not well understood. It is not clear how steps can influence the response of a planing vessel operating in waves. In the presented research, this problem is analyzed by developing a model, which is established using a 2D + t framework. Meanwhile, a CFD set-up is also designed to numerically replicate the motion of double-stepped planing hulls subjected to gravity waves. It is demonstrated the results of both models agree. Performing mathematical simulations, it is demonstrated that a double stepped design can decrease the heave and pitch responses of the vessel in the resonance zone. More importantly, mathematical data confirms that wave-induced motions of a vessel are highly sensitive to the heights of steps. A vessel with a shorter front step has smaller vertical responses, especially over the resonance zone, while it may have larger vertical acceleration at high-speeds. This is caused by the larger washed area of the middle surface, which can increase damping forces, while it allows larger wave forces to impact the vessel.