We develop a renormalization method for the quasienergy spectra of low-dimensional structured systems under intense ac fields. These systems are emulated by tight-binding lattice models with a clear continuum limit of the effective-mass and single-particle approximations. The coupling to the ac field is treated nonperturbatively by means of the Floquet Hamiltonian. The renormalization approach gives an intuitive view of the electronic dressed states. The numerical advantage over a direct diagonalization of the Floquet Hamiltonian makes the method suitable for the study of dressed states of nanoscopic systems with realistic geometries, irrespective of the ac field intensity. Two numerical examples are discussed: a quantum dot, emphasizing the analysis of the effective-mass limit for lattice models and double-dot structures, for which we discuss the limit of the two-level approximation currently used in the literature. © 2002 The American Physical Society.