Abstract Numerical and experimental studies of guided waves diffraction by surface-bonded obstacles of rectangular shape serving as a preliminary model for realistic stiffeners are presented. The simulations are performed within the semi-analytical hybrid approach based on the frequency-domain spectral element method and the boundary integral equation method employing integral transforms of Green’s matrices. The mathematical model is verified by comparison with the results calculated by the finite element method and with experimental data. The developed hybrid method allows investigating the scattering of each of the excited guided elastic waves separately, so along with the evaluation of diffraction eigenfrequencies, the conversion of Lamb waves at the obstacle is carefully analyzed. Analysis of complex resonance frequencies demonstrates that eigenfrequencies have large absolute values of the imaginary parts, that explains the absence of strong resonances. Nevertheless, eigenfrequencies with smallest absolute values of imaginary part pronounce themselves in resonance scattering of Lamb waves.